I've never been outside Essex/Herts so hard to say
The only places i've been to worth noting are:
1. Danbury: When it's dry it's super fun to just bomb down full pelt on my jump bike
2. Chicksands: Again, good fun, smash it down the dual slalom against or mate of have a plat of the hip's and stuff over the other side, not forgetting the pump track either
3. Thetford: A bit too XC for me, although the "downhill" section of the black run is all right, haven't been there for ages though
Going to Forest of Dean uplift day on the 28th April, so i'm sure that will become my new favourite place