London Major Trauma hospitals:-
Royal London, Whitechapel; Kings College; St Mary's, Paddington; St George's, Tooting.
UCH, Guys and St Thomas's are not.
When I started at HEMS in 1999, the info given was that an ambulance on blue lights travelled at 17mph on average through the capital. The helicopter is used to treat the patient on-scene, and then is taken to the nearest, most appropriate hospital for their needs. This is probably less than 1 in 5 by air.