Is Mrs. Thatcher's style too imperious?

Are Prime Minister Margaret Thatcher's personal energies and ''conviction politics'' wrenching the British constitution out of shape? The constitution is unwritten and may therefore be plastic in the hands of determined politicians, but the question is by no means academic. It is being asked by some of Mrs. Thatcher's own Cabinet ministers.

Edward Du Cann, chairman of an elite committee of Tory politicians, has dared to opine on television that Mrs. Thatcher is working too hard and should appoint a deputy.

For a long time, concern about her seemingly imperious style tended to be confined to civil servants bruised by her impatience with Whitehall bureaucracy.

After the Tories' big win in last June's general election, Thatcher abolished the staff of professional advisers who rendered independent counsel to the government and began building up her own policy advice group at 10 Downing St.

Tory critics claim the new group, responsible only to Thatcher, expands her power at the expense of the House of Commons. Government ministers say the Cabinet itself has been diminished in its functions.

Her ministers work on a range of Cabinet committees dealing with such matters as defense, industrial relations, and economic policy. But Thatcher has set up groups of advisers whom she believes she can especially trust. This has concentrated power in her hands, since she determines the membership of such groups and usually chairs them.

Mr. Du Cann argued that the government had made a series of errors lately because Thatcher was ''working too hard.'' What he meant was that the prime minister had made radical adjustments to the machinery of government that had helped to produce ''banana skins'' - slip-ups on key points of policy.

Thatcher says she has no intention of reducing her workload or appointing a deputy. Nor does she plan to curb her personal think tank or rely less on ad hoc ministerial advice.