ON OR ABOUT July 28 two Rolls Royces, accompanied by a police escort, will make the 105-mile journey north from Bournemouth, on the southern coast of Britain, to London. Destination . . . Buckingham Palace.

Each of the Rolls will be carrying two tiers of an 8-foot-high, 120- to 200-pound wedding cake (plus repair kit, spare parts and a bag or royal icing), a gift from Mary Ford Cake Artistry Centre Ltd. to Prince Charles and Lady Diana Spencer in honor of their marriage July 29.

Despite its cost -- almost $7,000 -- this wedding cake will be only one of many the royal couple receives. Despite the 1,500 hours of labor that go into it, the cake will not be the one cut by the couple at their wedding breakfast following the 11 a.m. marriage ceremony.

(Why a meal served at high noon or later is still called a wedding breakfast remains one of the many mysteries of wedding protocol. Prince Charles' wedding breakfast probably will be a three-course meal, each of the courses accompanied by its own wine. Champagne will accompany the cake.)

Mary and Michael Ford of Bournemouth own just one of many bakeries following this tradition of uncertain origins and age.

It's customary for bakers around the country to offer cakes to the royal family for special celebrations: there were 15 or 20 for Princess Anne's wedding in 1973, six or eight when the then Princess Elizabeth married the Duke of Edinburgh in 1947 even though such essential ingredients as butter and sugar were still rationed. The Queen Mother's Jubilee last summer was another occasions for cake offerings.

"Normally," a spokesman at Buckingham Palace explained, "the royal family doesn't accept gifts from people they don't know or from commercial firms. But when it come to something like a royal wedding, about 80 percent of the gifts are touchingly made small things, and it would be hurtful to return them. So the general rule is the presents are accepted."

They are accepted on the basis that they will not be used for publicity or advertising. "Curiously, most people who offer a cake want to do it for sheer niceness. Oh, I'm not saying they wouldn't have a picture taken of the cake for a local paper," she said.

"Ordinarily, manufacturers do not use the names of members of the royal families or pictures. They would be pounced on by fellow manufacturers. Almost universally people are very good about this. At the time of the royal wedding, these conventions are relaxed, and guidelines are set out for their use."

Indeed, these restrictions are quite clear in the letter that the Fords received after they wrote the palace and asked to submit a cake. The reply from the master of the palace household accepting their offer said: "I must ask that you will not use the fact that you will be supplying a cake to His Royal Highness in any advertising of your products. A small press release is perfectly in order," he said.

So far, BBC, Austrialian television and The Washington Post have been down to Bournemouth to interview the Fords. On the day of the wedding, Michael Ford intends to take out full page ads in the papers thanking all the people who donated services for the cake: the Rolls Royce dealer; the Bournemouth police and the persons who donated the ingredients-butter, flour, eggs, marzipan, brandy, rum, etc. "Free publicity," said Michael Ford, "if they get on the bandwagon. It will be a hell of a day. We need the Rolls Royces so the cake will have a smooth ride. We must have a police escort because we can only go 30 miles and hour."

Just in case a bump or two damages the icing, the Fords are taking along some spare panels and extra icing to make repairs.

"We always said we wanted to make his cake just for the pride of it, me dear," Mary Ford told a reporter. "I suppose it was my ambition. If other people can do it, why not Mary, I said to myself. Everyone in the shop laughed, me dear, when I wrote the letter. And they laughed again when the palace answered."

Mary Ford says she is making the cake because of ther admiration for the royal family. "They do a good job. Why, Prince Charles can't wipe his nose without someone taking a picture of it," she said, referring to his spill at a steeplechase race earlier that week, when he ended up with a bloody nose. Ford was sitting at the back of the shop where a class in cake decorating had just ended, but her eyes were on the front where a few of the ladies from the class were still making purchases.

The man who is helping the Fords design this four-tiered piece of gothic architecture in royal icing, also helped to create a cake presented by a Scottish firm to Princess Margaret when she married Antony Armstrong-Jones.

Thus far, only the design of the bottom layer has been complete. It will carry the royal seal of the Prince of Wales, cherubs, flowers, untold numbers of curlicues and the initials "C" and "D."

The Fords opened their bakery 10 years ago. Calling from London, it sounded like a mom and pop operation. Instead, a reporter found a shop where the Fords teach 400 people a week. Last year it grossed more than $650,000. They make five tons of wedding cake each year and have 80,000 customers who buy their cake baking and decorating equipment as well as cake decorating lessons by mail. The Fords -- Mary does the teaching and handles the cutomers, Michael handles the mail orders and the money (I'm not interested in making a bun, I'm interested in making money. She speaks sweetly to the old dears. I can't," Michael explained) -- are anxious to share their wedding cake plans with the world. Other bakeries are less forthcoming.

Lyons, a firm that caters the queen's garden parties, is one of them. "A cake certainly will be made and presented," said a spokesman. "We are," he sniffed, "the only holder of a royal warrant for making cakes." (Royal warrants appear on English products as "purveyor to Her Majesty, Queen Elizabeth.") "We will have some idea at the end of May what the cake will be like. Basically it will be a traditional cake, a darker fruit cake," he said, but he would say no more. Until Princess Anne's wedding in 1973, either Lyons or McVitie & Price (who made Queen Elizabeth's cake), had invariably suppled royal wedding cakes.

A well-known and expensive London caterer, Floris Gruhn Ltd. said, "We do intend to be making a wedding cake for Prince Charles. It's part of our usual tradition.I cannot say more about it," the spokesman added, "because all of the bakers try to outdo each other."

The cakes themselves are basically all the same: eggs, brown sugar, raisins, sultanas, candied fruits, nuts, spices and spirits. It is the decorations that set them apart.

Some firms determinedly set themselves apart from the custom of offering cakes. "Oh God, no," said a spokesman for Searcy, a firm which caters to the Mayfair crowd. "We wait until we're asked," he said, horrified at the thought others woudn't. In the presence of royalty one doesn't speak unless spoken to, either.

There is some gossip among bakers that the official cake may be made by the chef aboard the HMS Bovington, a battleship on which Prince Charles served. Or perhaps the Navy Cookery School will be chosen. This speculation is based on the fact that Princess Anne's wedding cake was made by the Army Catering Corps because her husband, Mark Phillips, was in the army.

The Navy Cookery School says the matter hasn't been discussed. We consider ourselves highly qualified. The Army's good, but I'd like to think we're better. But we wouldn't take the initiative."

It is unlikely that the palace staff of 12 to 14 chefs and cooks, including a pastry chef, will make the cake because the work is too specialized and too time consuming. But the wedding breakfast will be their work.

Although there will be 2,700 guests at the ceremony, somewhere between 120 and 140 will be invited to the palace for the breakfast if the weddings of Queen Elizabeth and Princess Anne were typical. Before the cake is cut, the guests will dine on the three-course "breakfast," probably fish and game followed by an ice cream bombe. This is a description of Princess Anne's wedding breakfast by a palace spokesman: the fish course, Oeufs Drumkilbo, "chopped hard boiled egg and shrimp in a pinky sauce; perdreau, that's partridge, isn't it, and veggies, then a bombe glace royale, some sort of ice cream pudding." The meal was accompained by wines from the royal cellars: '69 riesling, '55 Chateau-Mouton 'rothschild and '66 Heidsieck champagne.

"If I had to guess," said Anne Wall, one of the three press secretaries to the royal family, "which I am always reluctant to do, Prince Charles' will probably be very much the same."

In addition to the official cake, the other cakes are expected to be on display at the breakfast. "To the best of my knowledge we don't say no to anyone who wants to submit a cake," Wall explained. "But if hundreds of cakes are offered, I don't know what will happen."

After the wedding they will be "all chopped up and sent to old people's homes and children's organizations," she said.

The top of the official wedding cake will be set aside, not for the first anniverssary as is the American custom, but for the first christening.

What that cake will look like, the menu for the breakfast, the wedding dress are all closely guarded secrets. They are unlikely to leak out unitl the palace is ready to release them.

Despite the fact that the Fords' cake will go under the knife, its design will not be lost forever. They are making a replica because "local people are so interested." It will take an additional 500 hours, using wooden frames as the base for each layer.

The Fords have had 40 requests so far for the same wedding cake -- that's the cake itself, not the royal icing design. "We would make it, if someone asked," Michael Ford said . . . and had $6,900.