Compared to JFK, IAD and some other US airports, LHR is by far cleaner, more modern and more comfortable. I have been traveling in and out of there since I was 12 years old, unaccompanied, so it can't be that confusing.
Security guards are not meant to be smiley, shake-your-hand, have-a-nice-day people. They are meant to be there to enforce security. After years of dealing with Irish terrorists, American tourists, British soccer players and German beer-drinkers it's no wonder they're a little somber. Don't forget as well, that in Britain you don't say hi to everyone you meet. You let people do their thing and you do yours. I have found TSA agents a lot more unfriendly, uneducated and generally more malcontent than any foreign security agent....except maybe French customs.
Ray