Do you think maybe they're shifting some of the US immigration functions over to the departure ports? I also fly a fair bit and never had anything as thorough. I think I'd be a bit freaked out at being examined like that before departure.
No, again, it is private security hired by the airlines or the airport operator. On the back of my passport I have 7 little stickers that all came from Heathrow and United Airlines quizzing me on departure.
I was also stopped once by the police at Gatwick to ask me some questions. It seemed to be a random thing, but they took my name and address and got me to prove who I was. Also, since I fly domestically in the UK a lot as well at Gatwick they take pictures of the domestic passengers to match up when they go to their gate. Well, it would seem that once they took the picture of the guy behind me and not me and had to call security over for me to prove I was who I said I was, why was I flying, etc. Luckily I showed up at the gate early, because it took like 30 minutes to go through.
My father's name "Patrick Kelly" appears to be on the US no fly list. So whenever he flies, he gets interrogated to prove that he isn't the Pat Kelly they are looking for.
I had a friend once who looked like an IRA terrorist. It would appear that on arrivals from NI and the ROI (when you pass in front of those big glass windows at Heathrow) they had some form of facial recognition stuff, and they would often come out and interrogate them. One time they came out with dogs and made him drop his back as the police dogs sniffed his luggage and crotch.
The only overseas I have seen has been in departure via Dublin, and there you clearly know it is US Immigration.