mutt: "Have flown through terminal 3 over 100 times" Gee, is
that why it has all those big holes in it?? Couldn't you just travel through it like the rest of us.
I have never been stopped by customs when passing through the Green channel at LHR. When I have stopped in the Red and declared goods, I have been treated with great curtesy. Once, in T2 he got awkward but that was 16 or so years ago and the only time.
With Immigration in the US, I was pulled out of the queue at JFK one time (March '03, I think) and given closer questioning and my bag gone through but it was a 'light' examination and I was on my way. They behaved well.
As with all of these groups. Do your best from the start, even if they do not. You can be 99% certain that it's on CCTV and if you are shown to be 'reacting' from the start, they are going to crawl all over you. They may also be tired and hot and uncomfortable but don't give them any reason to be unreasonable. In the USA, they need no invitations and WILL be horribly 'firm'.