DITCHLEY PARK, ENGLAND -- For 350 years, the estate here outside Oxford belonged to the same Lee family whose descendants placed their mark on American history. For the past 25 years, the ''new'' manor house (dating only from the 1720s) has been the site of Anglo-American conferences on topics of importance to Great Britain and the United States.

Last weekend, at the third such session I have attended in the past 15 years, the subject was about as basic as you can get: the interplay of public opinion, the press, politics and government in our two democracies.

If you think that sounds impossibly academic, let me set you straight. The 30-odd politicians, pollsters, professors and journalists collected by the Ditchley Foundation and the Chicago Council on Foreign Relations had to ponder parallel real-world paradoxes.

Britain, with its strong party discipline in a parliamentary system, permits Margaret Thatcher's Conservative government to make fundamental -- even radical -- policy changes with a political base of only 43 percent of the voting public.

The United States, with its weak political parties and separate legislative and executive branches, denies effective and enduring power -- and accountability -- even to a president like Ronald Reagan who has won two landslide victories.

The chronic British problem is incipient parliamentary absolutism. The chronic American problem is incipient deadlock between president and Congress.

''The Ditchley Solution,'' as several veterans of past conferences noted, is almost always to suggest that each country could benefit from a touch of what the other has in excess. Britain could use a little more flexibility in the joints of its government, to make commanding figures like Thatcher heed dissent and seek a broader consensus for her policy changes.

The United States could use a lot more backbone in its parties and some means for forcing agreements between the White House and Capitol Hill on basic issues from the budget to Persian Gulf policy.

The press plays a major role in all this. Polling is a relatively new art, and even the concept of public opinion has not been around that long. But both have come on at express-train speed in large part because of the press' fascination with them.

In the recent British election, practically every newspaper and most of the major television news programs sponsored individual polls. Ditchley rules do not permit direct attribution of statements, but a highly knowledgeable British pollster told the conference that the reason publishers love polls is that they mean cheap publicity: each new poll is reported by every other news outlet, so the name of the sponsoring organization is repeatedly mentioned.

The polls steered the election dynamic. The Conservative share of the vote barely budged from beginning to end. But as Labor and the Alliance of the Liberal and Social Democratic parties battled over the other 57 percent, crucial to the division of seats in Parliament, the polls took on a life of their own, drowning out debate on the substantive policy differences among the parties.

Britons at the conference raised the possibility of banning or limiting public polls -- and just as quickly discarded it as impractical. But the clear sense we got was that the hype of polling by all parts of the press had just about driven British voters -- and politicians -- to distraction.

The situation on our side of the Atlantic is, if anything, worse. Some of the American pollsters tried vainly to play down their role. We in the press did our usual unconvincing job of denying that we have any real influence on politicians or government officials. It's a ritual we've learned so well we can't break the habit.

But in reality, the play of public opinion, as reported and magnified by the press, has grown so powerful in the United States that it has become the near-preoccupation of government. Someone said here, quite correctly, that when a president loses popularity, he also loses the ability to govern, whatever the Constitution may say.

That's the problem facing Ronald Reagan these days. It's ironic in a way, because no president ever worked harder to maintain his grip on public opinion than this one.

After the Reykjavik summit last autumn and after the recent economic summit in Venice, both of which saw American leadership go awry, the Reagan team went to work, not primarily to repair the damage or adjust policy, but rather to convince the American people that setbacks should be seen as victories.

The priority that presidents give to image-making is equaled or exceeded by most members of Congress. And presidential candidates these days do hardly anything but preen in public for two years or more.

This Ditchley conference served as a sobering reminder: while public opinion must be the foundation of a representative democracy, the worship and manipulation of the polls cannot substitute for the hard work of governing.