Andy Aitken of the Georgetown Prep soccer team looked out the window of the British Airways 747 at London's Heathrow Airport yesterday morning, saw a blue and white jet with the words "United States of America" emblazoned on it, and wondered along "maybe that's the president's plane."

It wasn't , but Aitkin, who was returning with his teammates from a two-week playing tour in England and Belgium, can be forgiven if he mistook the jet that was taxiing nearby for Air Force One.

When Aitken saw was an Air Force 137 (comparable to a commercial 707), which is part of the Special Inter-Mission (SAM) fleet that is stationed at Andrews Air Force Base. It is the jet that has been hopscotching around western Europe for 12 days with the American delegation to the Interparliamentary Union.

The 14 members of Congress and more than twice as many wives, aides and military escorts returned to Andrews yesterday afternoon by a somewhat more direct route from Lisbon, site of the IPU's spring meeting, than it took going to Portugal in the first place.

The delegation had made side trips to London, Paris and Geneva, in addition to the meeting site in Lisbon, during the course of the trip.

Arthur Kuhl, the assistant Senate secretary who acted as the delegation's purser, stood in the duty-free shop at Heathrow yesterday shortly before the takeoff of Air Force 137. He was complaining about the inflated prices - silk French scarves at $50, men's tiles at $15 and up - and said of the $45,000 appropriated for three IPU trips this year. "I think we shot the whole thing" on the first trip.

A good chunk of whatever was spent - exact figures weren't available yesterday - went toward room and board in one of London's finer hotels.Delegation members, including Senate Foreign Relations Committee staffers and other Hill aides, stayed at the Churchill Hotel in London where a single room costs about $80 a night (compared to about $30 a night in Lisbon).

Breakfast in the room there cost about $6, and a number of those who made the trip availed themselves of that room service option at government expense.

But no one in the 46-member delegation yesterday could say just who was responsible for picking the accommodations, deciding how many aides should come, or even why part of the delegation left Lisbon before the IPU meeting was completed.

It was left to "the chairman," Sen. John Sparkman (D-Ala.) to respond by saying, "It's always been done this way."

Sparkman said "two or three" staff members of his committee usually accompany the delegation, but that individual senators and representatives may recommend that other staffers come along.

The entourage this year included 13 congressional wives, a staff of 11 - plus three of their wives - and five Army escorts in addition to the 22 members of the jet's crew.

Sparkman said staffers are not selected for the trip as a reward, but because they are needed to assist the congressional delegation.