Flight paths over London have been there for a long time. They are only noticeable on clear days when viewed from below, and if they leave condensation trails.
As I understand it, flight paths are typically from North America to/from Southern Europe for long haul. They are at 35,000 feet or greater (number of variables). I recall when commuting to Munich that we overflew Paris at high altitude. Same thing with London, we just happen to be on the best circle route for some city pairs.
For short haul, a map can show some obvious links that will also see the aircraft above 30,000 feet for the transit of London.