So as far as I know, a flight from JFK to LHR would go from JFK Ground on either 121.9 or 121.65 (both on the JFK Ground feed), then the flight would switch to Kennedy tower (119.1 most of the time). Departure frequency is usually 135.9 (NY Departure JFK feed), but I've noticed that at night they combine departure with approach on 125.7 (ROBER sector). After this the flight is most likely handed off to Boston center...don't remember the frequency off the top of my head. As for EGLL, it is currently illegal to broadcast ATC feeds in the UK, so you won't be able to listen to any ATC from EGLL or from the rest of the UK, unfortunately.