The British male would not in ordinary times be caught wearing ''short trousers.'' But these are not ordinary times. Britain is suffering its hottest summer in 300 years, and knobby knees and hairy legs are in - even at the office. Stores have sold out of their tailored pin-stripe shorts. Even Savile Row tailors have been reduced to making shorts to measure for $210. ''The only people in Britain who wear shorts to work are Australians and South Africans,'' one Fleet Street worker said. But, it seems, the old ways are giving way.