Good move mate! I had my first go on a BMX at Rush Skatepark, Stroud last Friday night and was instantly hooked. After a bit of research and a max budget of £250 I’m now waiting on one of these to turn up (black 21″ one):
http://www.winstanleysbmx.com/product/43262/Haro_5001_2012_BMX_Bike
Check out the link below for all of their clearance stuff:
http://www.winstanleysbmx.com/category/263/Clearance_BMX_Bikes
Quite a few around the £150-£160 mark. As with all bikes you’ll get a higher spec (eg chromoly frame/forks) if you buy second-hand, depends what you want and how long you are willing to wait.
If you’re over 6′ try and look for a top tube of 21″. Normally they’re between 20 and 20.5″.
I enjoyed it so much, can’t ever remember getting such a buzz from MTB’ing. I’m 39 by the way… mid-life crisis ahoy!