I've lived for more than a year in both Berlin and London, flying a lot to and from their respective airports. The difference is indeed huge. I've never had any major issues personally, but I know several people who've had major problems (detained for hours and invasive body search, etc) at London airports. When I left my job in London a few months ago it was such a relief to know that would probably be the last time for a long time I had to spend time at one of London's airport, LHR/LGW/LTN/STN. The only pleasant one is LCY, which is really nice but unfortunately quite local (lots of Swiss flights).

Whenever I land at TXL or SXF in Berlin there's a sense of liberty. They are both very primitive airports, but I literally feel the absence of Stasi there, at least for now. People in Berlin have quite an intimate and visceral relationship with surveillance due to their recent past, so you can feel the counter forces of that in the atmosphere.