I have to say where I live in Somerset is pretty good - two indoor arenas within 30 mins, 3 BE XC within 45 mins, two of them have have lots of events, 2 other xc within 45 mins, gallops across the road from me, Hill's within 20 min hack, Beach either 10mins box or 45 mins hack and even better so close to M5 for getting to comps.
Devon- 5 mins hack onto Dartmoor down a really quiet lane then thousands of acres with right to roam - 45 mins to several different beaches 3 xc courses within 40 mins. 2BE events just 20 mins away 2 hours to london by train - return fare £35!!!!!!
After living in Essex and then Derbyshire, I now live in North Suffolk and it gets my vote. We are within easy boxing distance of the beach and also the forests. I have bridleways nearby and we have a local estate who have put a horse trail of approx 10 miles round their land which we can ride between Feb & Oct when they aren't shooting it. Most of the people who I see when I'm out hacking are really polite and friendly - they slow down in their cars or stop to say hello if they are out with their dogs. all in all, I'm pretty lucky I'd say!
My Oh works in london and if you book your tickets about a month in advance you pay £12 a single up to London and £23 a single back from London. He books about 10 sets at a time as soon as the cheap ones come up. It actually takes him less time to get from Exeter to London than one of his friends who lives south of the river! Also don't forget - the intercity trains are normally far more reliable than connex and the local trains and you can book a seat. Oh and House prices are cheaper than in the home counties.
If you want miles of off road riding and to be 30 minutes by main line train from Central London then I would suggest Hertfordshire in the area between Baldock Stevenage and Buntingford.
Property prices are still reasonable and there is a very good local Riding Club.