Iron Horse have some pretty handy dh / free ride bikes....
http://www.alpinebikes.co.uk have them in stock (i think they might be the importer in the UK for Iron Horse)
others to maybe consider are;
Honda ( if u have a spare £60000+)
Santa Cruz (steve peat / greg minnar)
Orange (although i have never rated them as any good)
GT (available through halfords (easy to get i imagine) but again i have never rated these)
This is the knowledge of someone who doesnt ride DH and never has, however i did work in alpine bikes a couple of years ago, and have had some training on them (iron horse), and they are renowned as very good bikes, after all they are ridden by a world beater in the form of sam hill....
EDIT: If you want tracks check out:
http://www.fortwilliamworldcup.co.uk/
http://www.7stanes.gov.uk/ (not sure if there is DH here though)
http://www.descent-world.co.uk/ (UK's DH home...so to speak)
http://www.southerndownhill.com/