In terms of travel time between main centres of population, and a very limited number of stops at lesser settlements, it'd probably down to:
London - Bristol (parkway) - Cardiff
London - Exeter - Plymouth
London - Liverpool/Manchester
London - Glasgow
London - York - Newcastle - Edinburgh
London - Leeds
London - Sheffield
London - Nottingham
That's about it. Most of the journeys are about 2hrs or more from their capital origin. Norwich is a marginal case as the intention at least is to drive the time down to 90mins. Birmingham is somewhere between a fast regional service and a pseudo IC service because it is on an IC route, (maybe in a few years faster services in the route will be a 200km/h 397 type of train, - the really fast services will of course be on HS2.