Coming into the UK through Heathrow, as a non-EU citizen, can be quite unpleasant. It's less so nowadays than before, and it was also highly dependent on where you are coming from and what passport you are carrying and when you are landing. That said, they tend to warm up the more you seem to know the drill and the more stamps for entry you have in your passport. For first time visitors the experience can be pretty aweful as they play Jeremy Paxman on you after you've just spent 10 hours on a flight. That said, if the immigration officer is of an ethnic minority they are usually much nicer. It's one of the times I don't feel bad for racial stereotyping, when I have a choice of desk after 40 minutes of queuing and aim for the non-white officer.

I find the other UK airports to be much friendlier in general, they even say 'please' and 'thank you' and even have a sense of humour.