Balaton
It is about a 6-6.5 hour, M.78 plane in executive configuration. Don't think it could do OBBI-EGGW unless favorable winds and weather in London area.
Strong points: Cheap, about Challenger 300 money with a longer cabin. Large baggage compartment, large cockpit with airline-style jump seat. Airline maintenance schedule and parts should be reasonable priced with all the airframes out there.
Weak points: narrow cabin, aisle curves around the seating in exec configuration. Restricted in maximum ceiling, I think FL390 should do it. Avionics are airline-related and somewhat out-of-date.
Overall, good deal for the money, if you need a 6-6.5 hour plane and can live with the cabin.
GF