Have to agree with Paxboy - 15-20 years ago UK Customs were abrupt, unpleasant and could be very arrogant and rude (like most Customs the world over)...certainly not the case generally now as they are very polite and do give the impression they'll keep you as little time as possible...
Cant say LHR T3 at any time has ever been any worse than any other UK airport in my experience