The first thing the British (and most other foreigners) urgently want to know from an American visitor in a time of slow-motion transition in Washington is what the Reagan administration is really going to be like.

And that's manageable: you can say it's hard to tell this early, and the questioner will understand. The next question is harder: why should it take so long to find out?

Or putting it another way: how can one of the two great superpowers, with all its responsibilities for leadership of the West, afford to take two-and-a-half months to change governments? After all, under Britain's parliamentary system, the visitor is reminded, when a prime minister is defeated, the moving vans show up at No. 10 Downing Street the following day.

True enough. If we had a parliamentary system of government, the Reagans would have long since moved into the White House, and we wouldn't be worrying, as a lot of people seem to be, about the protracted semi-paralysis of the current transition period. We would have had, these past four years, a Shadow (Republican) Government. Its leaders would have been sitting in the Congress/Parliament, participating in debates, voting on issues, offering alternatives and being tested every day. i

The Reagan Cabinet, if we can carry this just a bit further, would have been familiar faces, all their likely job assigments known to the voters before Election Day, their views well established. Most would have developed more than one area of expertise as they worked their way to the top. "Cabinet government," with everybody pitching into decision-making across the board, would make sense.

There would have been no transition.

But we don't have anything remotely resembling such a system. And so, in approaching the problem of how to improve the transition process, it is important not to pretend that we do.

The suggestion has been made by my colleague Tom Wicker, for example, that we put a one-month limit on the transition, moving Inaugural Day back to Dec. 10, and making government appropriations for transition costs available to the challengers by Sept. 1. It's an instantly intriguing idea.

It would leave the United States vulnerable for a far shorter period to a foreign crisis at a time of divided command -- with a lame duck in charge and the people's choice waiting powerless in the wings. It would allow candidates to start sounding out Cabinet choices and starting the time-consuming clearance process.

If the names leaked (as they inevitably would), so much the better: the public would have a better idea of what sort of government to expect from Candidate X. But right at this point, I see a problem.To the extent that a presidential candidate actually started handing out jobs during the campaign, there is a serious question of public policy, if not of law, in the offering of this form of "emolument" by office-seekers.

More important, however, is the sheer practicality of a candidate's trying to do serious transition work while in the crucial, closing stages of a campaign. Surely the same, close, trusted group of intimates who would be handling the campaign itself would also have to be deeply involved in transition work.

A transition period of roughly the current duration is to the presidency roughly what training camp is to a professional football team. There is more than enough evidence that, for the Reagan camp, it has been a learning experience -- about leaks, about staff infighting, about problems and potential crises it had never heard of, about the intractability of the bureaucracies, and even the ways of Congress. By Jan. 20, the Reagan crowd will be in better shape to take over than it is now, or would have been Dec. 10 -- even with a head start in mid-campaign.

There remains this matter of vulnerability to foreign threats or pressures -- and the uncertainty of the American response. But Ronald Reagan and Jimmy Carter have demonstrated an ability to work out discreet understandings on issues that can't wait -- the Camp David process, for one. As for uncertainty, it can sometimes have a chastening effect on those who would put the transition to a test.

In any case, a period of some uncertainty is inevitable. In short, our system requires a considerable time for transition -- for all the reasons that a parliamentary system does not.