Hi Jimmy. Good to hear you fancy getting stuck into trail riding. You need to ask your local group contact about lanes in Sussex - click on Groups on the TRF website, there is a link hidden right at the bottom "fully group details", click through and you'll find your Sussex contact. I'm part of the neighbouring South London and Surrey group, as far as I know parts of Sussex are completely devoid of byways you can used, but not sure about your exact area. But there are lots of lanes in Hampshire and Surrey. Check out the surrey website for tips on bikes.
http://www.surreytrf.org.uk/ (Lots of excellent advice on there, better than the main site.) I only passed my test recently so have a Yamaha DT125, which I love and which does the job, is light to pick up when you drop it etc, etc and you'll certainly get one for your budget. But you'll probably want to get a 250 or 450. Good luck and have fun!
Steven.