Yeah, LHR immigration has some intense questioning. I have been to a lot of countries, and they leave the impression as most invasive. (The rest of Europe is very pleasant. Hong Kong was my favorite; I felt like they gave 0 fucks about who comes into their territory. I've also been yelled at in Japan because I filled out some form in blue ink instead of black ink. The immigration agent gave me a long lecture about how "I know you can read Japanese because you had a student visa, what part of black ink didn't you understand?" I said I didn't have a black pen, and they didn't have any around. That seemed to be good enough of an answer. I guess the strategy is to rile people up so they say something dumb, but I guess I had nothing to hide except my taste in pens. And I suppose they want to test that I'm who I say I am; I should be able to answer that question in Japanese if I am who my passport says I am, though gotta admit this was about 10 years after that and I was pretty rusty! The mind games are pretty transparent, though tedious after an 18 hour flight.)
US immigration is also very invasive, but I have Global Entry now and haven't spoken to immigration in years. Love it. (Customs, yeah they'll still ask you questions. Immigration questions upon arrival to the US always annoy me because I feel like I have a right to exist in my country of birth and citizenship. If I have the wrong answer to some question, where are they going to deport me to? The US? But, I don't feel like I have any right to bring in goods from abroad without paying taxes, so if people ask me those kinds of questions, it doesn't bother me.)