that should be plenty of time. since you are transferring from ba's term 4, the buses run much more frequently than from term 3 for example. if you were on aa or ua, you'd be pushing your luck.

and the option of clearing customs versus using the bus...get real. when was the last time you got through a customs line in lhr from a us originating flight in under 1/2 hour? and they will give you major attitude if they find out you're only changing planes. don't check into the country for only an hour. that's a sure way to get felt up just like diana!!!