There is why one can over-fly LAX. Try asking them for a random routing that conflicts with either their arrivals or departures.
But you don't??? They are set routes to allow you to transit across the airspace, and I don't see why this could not be achieved in the UK. Is ALL the class A filled with airliners?
There are three ways to get over LAX now:
1) Shoreline / Hollywood park routes. ATC clearance required by Socal
2) LAX Mini Route. ATC clearance required direct from LAX tower
3) Special Flight Rules area. No ATC clearance required so long as you conform to their requirements (Sqwark 1201, less than 140kts, lights on etc......)