How much flak can British Prime Minister John Major survive? Among his several liabilities: His Conservatives recently lost a House of Commons by-election and control of 15 of 16 local councils. The Conservatives are deeply divided over European union and economic policy. Rebellious backbenchers are openly demanding that Major dump Chancellor of the Exchequer Norman Lamont for proposing a 17.5% tax on home heating oil. Major is under pressure to reshuffle his Cabinet, and the Prime Minister could face a serious leadership challenge by fall.