IT'S EXTREMELY UNFAIR, the senators say, to accuse them of having fun at the taxpayers' expense in London. They were just passing through on their way to the arms control talks in Geneva. It was pure business. That's why they were traveling in an Air Force plane rather than by commercial airlines. That's why they had to be met at London's Heathrow Airport by U.S. Foreign Service officers and taken in state to their hotel by embassy cars. That's why four of the five senators had their wives with them. That's why they needed free time to go shopping in the London stores. That's why they had to have the embassy staff make dinner reservations for them, and get tickets for "Les Mise'rables."

The delegation, for the record, was headed by Sen. Claiborne Pell, the chairman of the Foreign Relations Committee, and, of course, Mrs. Pell. It included Sen. and Mrs. Ted Stevens, Sen. and Mrs. Don Nickles, Sen. and Mrs. John Glenn and Sen. Richard Lugar. This visit, 10 days ago, was not out of the ordinary. To the contrary, it was objectionable precisely because most members of Congress think that this kind of travel is one of the perquisites of their rank -- and one of the offsetting benefits of attending meetings on harsh and unforgiving subjects like arms control. As for the Foreign Service officers whose weekend was spent shepherding the delegation, Sen. Stevens huffed that "it frosts me to have guys sitting there on easy street in London" and having the temerity to think that there might be better ways for them to spend their time.

Meanwhile the State Department is going through drastic budget reductions. Consulates are being closed, spending is being fiercely squeezed, and there are layoffs ahead. The State Department is going to be required to do an expanding job with less money. Some of its traditional duties are going to have to be neg-lected.

But not, it seems, the very traditional duty of providing American diplomats to serve as tour guides and footmen for congressional visitors. It continues to be remarkable, and sad, that people who here in Washington talk endlessly about the need for less federal spending and better public priorities still expect and require treatment on a truly ducal scale while on official shopping trips abroad.