I'm in the process of getting mine at the moment, I've got the 'go racing' pack, had the medical and my ARDS training/test is now booked.
Basically, buy the pack, it'll tell you the rest -
http://shop.msauk.org/product_details.asp?id=4023
Have a look through the other sections, but I'm guessing that's the one you're looking for. The basic process is (explained in more detail on the site, and in the pack) :
* Get the pack, fill in the details, study the 'blue book'
* Get a doctor to handle the medical section (I got one done for 45 quid)
* Take the form with the medical section filled in to an ARDS centre. (I'm going here:
http://www.silverstone.co.uk/php/ma_ARDS.html). There is a practical and written test. My understanding is it's not too tough as long as you don't act like a moron, but I've not taken it yet.
* Once you have your practical done, send it all off with yet more money, and get your 'national B licence'.
All done!
Now, to actually race, budget another 1500+ for saftey gear, and god knows how much for your chosen vehicle. There's plenty more to be paying for too (entry fees, consumables) but hey, anything worth doing . .