THIS Wednesday, at the ungodly hour of 8:30 a.m., about 250 guests will descend upon the British Embassy, guests of the Ambassador and Lady Henderson, for a breakfast "to celebrate the Royal Wedding," as the invitation reads. An accompanying card informed the recipients that "guests at the breakfast will be able to watch television coverage of the wedding ceremony and procession (both live and taped)."

Unlike the royal couple's own wedding breakfast, which in reality will be lunch, the celebration at the embassy here will be a proper English breakfast, followed by a proper English wedding cake, accompanied by proper hot and iced tea (Lady Hendersonhs own and elegant recipe) and Buck Fizzes, which those in this country call "mimosas".

One problem that has demanded a good deal of thought on the part of Lady Henderson and her small staff is logiustical, or where to put enough television sets so that the live and taped coverage can be seen by Vice President and Mrs. Bush, the Casper Weinbergers, the Edwin Meeses and representatives of the British Commonwealth countries (for whom national dress is optional), of NATO and the European Economic Community, along with assorted other government and Washington types.

Then there is the food. Two weeks ago, Lady Henderson sent Chef Harry Simpson a memorandum officially aerting him to the forthcoming festivities and the kitchen's responsibilities. For the breakfast, the chef and his two aides, Jean-Claude Fostinelli and Ian Knox, will served the food from a long buffet. An astonishing portion of the cooking will have taken place minutes before the guests arrive. A minimum of 300 eggs will have been scrambled in bain maries (over hot water, see recipe), according to Lady Hendersonhs instructions; 80 pounds of bacon will have been cooked; 100 pounds of sausage and 250 tomato halves will have been grilled, as well as 250 halves of kippers that were smoked in Scotland and trucked down from New York; countless slices of bread will have been toasted and 20 pounds of cheese will have been melted to make the welsh rarebit. The orange juice, grapefruit sections, very small iced cakes and petit fours will have been prepared in advance, along with the iced tea. Jean-Claude Fostinelli, formerly of the American Embassy in Paris, will also have made the croissants and have them ready to pop into the oven before the guests arrive.

The wedding cake, a three-tired beauty, will be Chief Simpsonhs creation. The cake will be decorated according to Lady Henderson's instructions, with red, white and blue ribbon that will cover the boards which support each of the layers, as well as with small silver slippers and hearts and pink, blue and white flowers. On the top layer will sit a little crown the chef fashioned of confectioners' sugar, gelatin, cornstarch and water and the three feathers brought from England that are the symbol of the Prince of Wales. Separating each layer will be the pillars that rendered the same service for the wedding cake of Lord and Lady Moore, the Henderson's daughter and son-in-law.

The layers are dark and rich, heavy with dried fruits and brandy; an un-iced they were 2 1/2 inches deep, the three layers measuring 8 inches, 6 inches and 4 inches in diameter. Finished, their diameter are 10, 8 and 6 inches respectively. They were baked by the chef over three months ago and he has since laced them every four weeks or so with a brandied syrup, a trick of Lady Hendersonhs which she feels works much better than brandy alone, to make them mellow and keep them moist and fresh. Last Monday, the chef enveloped the tops and sides of each layer with a half-inch-thick sheet of almond fondant icing, a kind of marzipan which is rolled out and then applied onto the cake. (The cake is first brushed with melted red currant jelly so the marzipan will adhere better.) After four days, during which the marzipan dried out sufficiently to become a hospitable base for the royal icing, the three-day process of frosting the cake began. (A little glycerine was added to the royal icing lest it dry to a hardness of mortar.) The first coating was heavy, a guarter of an inch thick; the second was just slightly thinner, and the last was paper thin. The purpose of these steps ws to fill in any air bubbles and assure a smooth and very white base for the final decorations.

For these, the chef took out his pastry bag and tubes and piped on the pink, blue and white flowers and then applied the silver slippers and hearts. Tomorrow, when the decorations are firmly set, the cake will be assembled. Chef Simpson will place the bottom tier on a silver stand, and then he will fit eight three-inch-high pillars within the circles of flowers that he made as their base. He will then place the second tier on the pillars and will be placed on these and then the crown and feathers will be set on it. Finally, the ribbons will be applied to the boards. The cake, with its decorations, will stand about 34 inches high. Then, on Wednesday morning very early, the cake will be placed on a beribboned buffet table, a centerpiece fit for a future king and queen.

This delicious, very English wedding cake can also become, with different decorations, a traditional Christmas cake or a birthday cake.

THE MENU: Orange juice, grapefruit sections; scrambled eggs; welsh rarebit; bacon, sausages; grilled Scottish smoked kippers; grilled tomatoes; toast and marmalade croissants; very small iced cakes and petits four; coffee, hot tea, iced tea, Buck Fizzes (orange juice and champagne); wedding cake. LADY HENDERSON'S SCRAMBLED EGGS (4 to 6 servings) 1 tablespoon butter 8 eggs 3 tablespoons heavy cream Salt and pepper to taste

Melt the butter in the top of a double boiler. Break the eggs into the pan and add the cream, salt and pepper. Place the top of the double boiler onto the bottom half, filled with barely boiling water. Beat the mixture with a wire whisk or wooden spoon until the eggs are cooked. They should be light and creamy, not dry. Should they become overcooked, add another yolk or a little more cream. LADY HENDERSONHS WELSH RAREBIT (8 servings) 1 pound grated sharp cheddar cheese 4 tablespoons butter 1 cup ale or beer 1 teaspoon dry English mustard 1 teaspoon cayenne pepper or paprika Salt to taste 2 dashes worcestershire sauce, or to taste

Use a double boiler, the bottom half containing barely boling water. In the top half, combine cheese and butter with ale or beer and dissolve over gentle heat, stirring in one direction all the time until the mixture is creamy and smooth. Then add mustard, cayenne pepper or paprika, a pinch of salt and the worcestershire sauce. Taste and adjust seasonings. Serve, preferably from a chafing dish, over toast. LADY HENDERSON'S ICED TEA (8 servings) 8 teaspoons Twinings' Earl Grey tea 8 cups boiling water Juice of 6 lemons Rind of one lemon, peeled in one piece, if possible Rind of 1 orange, peeled in one piece, of possible 1 cucumber, peeled and sliced Large sprig mint 2/3 cup sugar, or to taste 3 cups ginger beer (available in specialty food shops)

Heat a large teapot by pouring in some boiling water and then emptying the pot. Then add the tea to the pot and pour the 8 cups of rapidly boiling water over it. Let the tea steep for 6 minutes.

Put the lemon juice, lemon and orange rinds, cucumber slices, mint and sugar in a jug.Strain the hot tea over these ingredients and stir. When the tea mixture is cold, add the ginger beer and refrigerate until ready to serve. The previous recipes are adapted by Lady Henderson from "Mary Henderson's Paris Embassy Cookbook" PROPER ENGLISH WEDDING CAKE (Makes 11-inch single tier cake and serves 50) 1 1/2 pounds seedless white raisins 1 1/2 pounds dried currants 1/2 pound seedless black raisins 4 ounces glace cherries, cut in half 2 ounces almonds, blanched and shredded 4 ounces mixed candied orange, lemon and citron peel, finely chopped 10 tablespoons brandy Scant 4 1/2 cups all-purpose flour 1/2 teaspoons salt 1/2 teaspoon ground cinnamon 1/2 whole nutmeg, grated, or 1/2 teaspoon ground mace 1 1/2 teaspoons cocoa 1 pound butter Grated rind of lemon Grated rind of 1 orange 1 pound superfine sugar 1 1/2 teaspoons black molasses 8 eggs 2 ounces ground almonds

Prepare the fruit and shredded almonds and place them in a large bowl. Sprinkle half the brandy over them, cover and leave at room temperature for 24 hours. Stir several times so the fruits and nuts soak up all the liquids.

Sift the flour with the salt, spices and cocoa and them mix 1/3 of it with the fruit and nuts. Set aside. In another bowl, cream the butter until light, add the two grated rinds and the sugar. Beat with an electric mixer until light and fluffy. Stir in the molasses and then beat in the eggs one at a time. Fold in the ground almonds and half the remaining flour. Add the fruit mixture and combine. Then add the rest of the flour and mix well.

Grease an 11-inch springform cake pan and line with kitchen parchment paper or foil. Tie a collar of lightly greased paper around the pan -- it should stand two inches above the rim -- to prevent the cake mixture from spilling over.

Turn the cake mixture into the pan, smooth the top with a spatula, brush a little water over the top to help keep it soft during a long baking. Set the cake pan inside a larger empty pan (round or rectangular) to prevent the cake from drying out.

Bake in a preheated 325-degree oven for 3 hours and then reduce heat to 300 degrees and bake for another 1 1/2 hours. Check after the first hour of baking and when the cake is nicely browned, cover it with a double layer of kitchen parchment paper or foil.

When the cake is baked, let it cool in its pan for half an hour. Then carefully turn it onto a wire rack, but do not remove the parchment paper or foil. When the cake is completely cool, apply mellowing syrup, wrap it completely in foil and store in a cool place. The cake should be baked three months before the wedding. Mellowing Syrup Scant cup water 3 tablespoons sugar 2 tablespoons brandy or rum

Bring the water, sugar and brandy or rum to the boil over moderate heat in a large, clean pan. Cover and boil for 5 minutes. Remove from heat and cool.

The first time the syrup is applied, you will have to pierce the cake with a skewer. Then baste the cake with about 5 tablespoons of the syrup. Rewrap and store. Refrigerate the remaining syrup and then about every 4 weeks unwrap the cake and baste if with a few more tablespoons of the syrup. (Make up more syrup as needed.) Almond Fondant Icing

This almond fondant icing should be put on from four days to a week before the cake is iced with royal icing. 1 pound ground almonds 10 ounes superfine sugar 6 ounces confectioners' sugar, sifted 1 large egg 1 egg yolk Juice of half a lemon 1 tablespoon brandy or rum 1/2 teaspoon vanilla extract 2 tablespoons orange flower water 1/2 cup red currant jelly for cake

Place the almonds and both sugars in a bowl and combine. Whisk the egg and extra yolk with the remaining ingredients and beat into the almond-sugar mixture, using an electric mixer. Then turn onto a board and knead until smooth.

Roll the paste out to a thickness of 1/2 inch on a board lightly dusted with confectioners' sugar or between two pieces of waxed paper dusted with confectioners' sugar. The circle should be large enough to cover the top of the cake with enough excess to hang down over and cover the sides. Alternately, a round can be made for the top with a slight overhang; then a strip is rolled out and applied to the sides and the excess trimmed off.

Now dust your hands with confectioners' sugar and smooth the almond fondant icing firmly and evenly onto the sides of the cake. Turn the cake upside down, press it to flatten the icing on the top and roll a bottle around the side to give a clean, sharp edge. Turn the cake right side up onto a turntable and set aside in a cool, dry place for 5 to 7 days.

Just before the almond fondant icing is to be put on the cake, melt 1/2 cup of red currant jelly and spread it on the top and sides of the cake, pressing it into the cake with a spatula. Royal Icing 3 egg whites 1 to 1 1/2 pounds confectioners' sugar, sifted Juice of 1 lemon 2 teaspoons glycerine (available at drugstores)

Put the egg whites and half the sugar in a mixing bowl and beat with an electric mixer for about a minute. When well mixed, add the rest of the sugar with the lemon juice and beat until very white and peaks form.

If you are not going to use the icing immediately, cover the bowl with a damp cloth.

Three days before the wedding, cover the cake with a 1/4-inch layer of royal icing, smoothing it to make as flat a finish as possible. Refrigerate the