To get to the London Center frequencies should be very easy. Simply follow a specific flight as he climbs out. When the controller tells him to contact a certain frequency, just make the change with him on your scanner. Sooner or later you'll be listening to London Center.
At that point, stay on that frequency. You'll hear numerous aircraft at cruise altitude. To get more LHR Center freqs, keep listening. ATC will pass aircraft from sector to sector.
You'll surprised how well this works. Early in 2000, I drove from New Orleans to Denver, and by the time I got back home to N.O., I had the whole route mapped out by Centers (Houston, Ft Worth, Kansas City, and Denver).
Tom in NO (at MSY)
P.S. You can tell it was a pretty boring drive!!!
"The criminal ineptitude makes you furious"-Bruce Springsteen, after seeing firsthand the damage from Hurricane Katrina