I know that Weymouth & Swindon have done training days in the past. Do you have a local track where you can ask someone?
You occasionaly see bikes on ebay, although obviously you need to be careful not to end up with a pile of junk!
This is the place to look, you may see some adverts for equipment & training days etc (training / practice forum):
http://www.speedway-forum.co.uk