Grantham
Despite it's smallish size Grantham has quite a lot of places to ride. I haven't ridden in that many of the spots but they were all pretty good and there were lot's of rocks to practice natural stuff on. Just get out there and find stuff.
Best places to ride street are: the area behind B&Q. (there's a bit of derelict land with lots of drops etc)
Best places to ride trials: the post office, behing Marks and Spencer's, outside this garage place down the main street, the area behind B&Q, the Aldi carpark and there's a quarry up the hill past Aldi.
Grantham is just off the A1 and can be reached easily by train or car.