Bumpy near Leeds are probably your best bet
"Bumpy provides lessons in off road riding for children aged 6 – 16 on weekends and school holidays. We can do lessons on your own bike, or on one of ours. We provide overalls, boots and helmets, and our experienced Instructors can provide the best possible start to your life in off road biking. Starting with the basics on our tarmac training areas, you can learn all you need to progress onto our beginner, intermediate and expert areas of the Trials park."
http://www.bumpy.org.uk