For a striking number of Britain's most prominent politicians, this has been a summer to forget.

Prime Minister Margaret Thatcher, who has cultivated a reputation as indomitable, suffered a detached retina and underwent emergency surgery. Neil Kinnock, now considered certain to be the Labor Party's new leader, lost his distinctive, gravelly voice, then almost lost his life when he totaled the family car on a lonely highway late one night in July.

David Steel, the Liberals' leader, announced he was taking a three-month "sabbatical" to recover from what was variously described as exhaustion, depression and influenza.

Their misfortunes demonstrate the physcial pressures of public life, no doubt aggravated by the intensity of last spring's general election campaign.

Their personal troubles also reflect some other important uncertainties on Britain's current political scene.

Thatcher emerged from the election in a position of extraordinary parliamentary strength, and that remains undiluted.

But instead of the promising economic reports that accompanied her victory, the latest figures show that the long-awaited recovery is already slowing down and could even go into reverse.

The crucial trend of exports is lagging. At home, forecasters say inflation may be up to 8 percent again by next year; deeper than expected public spending cuts are now anticipated just to maintain a minimum version of the Conservatives' financial strategy, and unemployment continues to rise.

As a result of these and other indicators, the Confederation of British Industry last week scaled back its bullish prediction of growth to 2 percent for this year (a drop of one-half percent since June) and said growth would probably stop altogether a few months into 1984.

"That back-from-holiday feeling," is how the weekly magazine The Economist characterized the bad news.

The main political question is whether Thatcher will continue resisting demands to give the economy a boost, even though her calls for continuing sacrifice may seem gratingly hollow after yet another failure to sustain a revival. As for her troubled eye, that, she says, is "fine," although it remains bloodshot.

In the Labor Party, during the weeks since Kinnock's miraculous survival--he emerged with only scratches after his car flipped off the roadway and was flattened--support has consolidated behind him for the leader's job. He has the backing of a majority of the trade unions and local constituency groups, assuring him a comfortable majority over the other main contender for the post, Roy Hattersley.

Kinnock represents the party's dominant left wing and his greatest challenge, following Labor's worst election defeat in modern times, will be to convince the country that the decline is not permanent.

A bruising battle is expected for the position of deputy leader with Hattersley, from the moderate section of the party, a slight favorite over another leftist, Michael Meacher.

Personally, Kinnock is believed to favor an alliance with Hattersley to give the party much-needed unity and opinion polls show this to be the preferred outcome among labor supporters. Nonetheless, serious factional differences remain on crucial policy issues such as disarmament and economic strategy. The "hard left," for instance, insists that voters did not finally reject its radical policies, but only the way the policies were presented.

Kinnock, an engaging 41-year-old, is confident he can salvage the party. "Somebody up there likes me," he has taken to saying since the accident.

The case of David Steel is the summer's strangest political drama. The Liberals, whose pact with the Social Democrats produced a remarkable showing in popular vote, should be riding high. For the first time in 60 years, they (with the Social Democratic Party) have the real prospect of becoming the country's main opposition.

But as one political analyst observed, "This marriage made in heaven is not working on Earth." Many Liberals are reluctant to see their party merged or even permanently affiliated to the Social Democrats. With Steel supposedly resting, squabbles have erupted over procedural matters, prompting him to write an angry letter to the party's members of Parliament.

If the Liberals want "to potter about on the sidelines, I will be happy to remain a loyal member but not to continue indefinitely as leader," he wrote. The testiness of the letter, which chastised a number of individuals by name, astonished even Steel's friends and revealed that the party has much to do in the weeks ahead if the alliance with the Social Democrats is to fulfill its potential as a third force in British politics.

The Social Democrats' new leader, David Owen, is also known to be reluctant to merge the parties. For the moment, though, it is Steel's troubles that overshadow all else. "No wonder Owen is resisting the idea of a merger," wrote Bernard Levin in The Times of London. "Who wants to be a Siamese twin with a brother who insists on drowning himself?"