Is Idi Amin taking the route of William the Conqueror and the Spanish Armada, invading Britain by sea.

It's possible, and that possibility had Britain, Europe and Africa taking part today in a nervous watch for signs that the Ugandan president was going through with his threat to gate-crash a meeting of the Commonwealth nations here.

Uganda Radio announced that Amin, whose country is welcome in the Commonwealth but whose personal presence is not desired because he is regarded here as a mass murderer, had left on the airplane of a "friendly country."

He would fly to a countty near Britain, Uganda Radio said, and then enter by boat.

That set off a frenzy of activity in Britain, which was having a good time celebrating the 25th anniversary of the reign of Queen Elizabeth and did not want to see it interruptes, and a flurry of reports on Amin's alleged whereabouts from many places.

Britain immediately put all air and seaports on maximum alert.

Amin's plane was first reported over Europe by Rome air-traffic authorities. Airport police reported sighting a plane accompanied by several fighter planes of unknown nationality flying over Calais, France, in the direction of Belgium.

Hours later, the Irish government announced that it had been alerted that Amin was approaching Dublin's airport and it quickly ordered that he not be allowed to land except in an emergency. BBC reported forthwith that extra police and an undisclosed number of Irish army troops were rushed to the airport.

But Irish officials said they had made neither radio nor radar contact with the plane, which was subsequently believed to be headed back toward the European mainland.

Despite the alacrity of their response to the report of Amin's coming, British security officials were skeptical that he was really on his way to the Commonwealth meeting, which is to open Wednesday.

"Amin likes to keep us guessing," one aide said. "He likes to play cat-and-mouse games. But we don't think he's coming."

Britain's Prime Minister James Callaghan has made it clear for some time that Amin would not be welcome at the meeting and Commonwealth leaders appeared to back his decision.

Amin first said he would not attend the conference, and there was a collective Commonwealth sigh. Then came today's announcement from Uganda.

If Amin does show up at a port here, security officials said, he won't get far inland. Customs officers have been told to treat him with the deference due a head of state and politely order his to a private room. A Foreign Office official will then be dispatched to tell Amin he is unwelcome and offer him swift transportation home.

Last night another report from Uganda said that Amin had reached his first destination, and unnamed Arab country and would sail to Britain Wednesday or Thursday from France, West Germany or Ireland. His boat would be "escorted," the announcement said.

Radio Nairobi, however, introduced a note of realism into these surreal fantasies. It pointed out that Amin's bodyguard and press spokesman were still at home.

The Kenyan radio, no friend of Amin's refused to believe that he would leave his country without these vital adjuncts.