I've done Level 1 and 2 GS courses with BMW Offroad Skills and they were excellent if expensive. They are good full days out on the bikes with experienced instructors who are all friendly and eager to get you riding to the best of your ability.
I learned a massive amount and enough to take me from offroad novice to riding my Tenere across Mongolia. Every little thing they taught me kept going through my head as we crossed.
There is also The Adventure Bike Warehouse in Swinton, Manchester, where Gaz the owner can take you out on one of the shops 250cc Hondas for a days training. If you have a bike that needs serviced they'll do it while you are out training...they call it a "dirty service"! Check out their website.

Hope that gives you some help.

Sandman.

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2011 XT660Z, in BLACK of course!
Mods done:
Lost the will to keep track!