Easy. EGLL/EGKK/EGCC. And for the simple reason that I grew up, both personally and professionally, in the United States. KJFK/KORD/KBOS make sense to me. And I have fond memories of KDCA.
Everyone is more comfortable on home turf. Every country as its own aviation culture, notwithstanding ICAO. And almost everyone has to find some reason to look down on other people. These are, I think, three facts of life.
But neither is my home turf.
I have never received an uncomfortable clearance in London without it having being first offered as an option. Manchester can leave things tight but it is a rare occurrence and a request for extra miles would never cause a problem.
The three US airports I mentioned have frequently given me instructions with which I was not comfortable.
IMHO the big difference between the two is not ICAO. It is not ability or training. It is simply respect.