Depending on what you want to spend, if you want good gear on a cheaper frame ive seen some good deals on Dawes XC and do not forget the Halfords bikes i.e. Carrera which seem to always be on sale but come highly recommended by many on Bike Radar because of their Gear to Cost VFM factor.
You have two choices either Shimano or SRAM now for trail riding, Shimano Deore is sort of midrange, but you wont find it much on cheaper bikes due to the recession, you might end up with Altus or Alivio if you spend a few hundred quid, alternatively you could aim for SRAM 5.0 which can be found on well priced bikes.
You should also look for forks with a decent amount of travel, the low end stuff is under 100mm, try to get some with 100-120mm of travel and look for the feature "Lockout" which basically means you can Lock them from bouncing up and down when your on the road, otherwise without that feature your just wasting energy for the sake of it.
As for disc brakes, on the cheaper (decent branded bikes) you will get either mechanical brakes (if its a cheaper brand) or maybe even V-Brakes on a premium brand because your paying for the name not the specification of the equipment setup, for a little more on the cheaper (decent branded bikes) you will get hydraulic disc brakes, much better in the long run.
Ill link a couple ive had my eye on recently:
Carrera Vulcan £329.99Carrera Vengeance Ltd. £329.99Carrera Kraken £429.99The Kraken is what i would go for personally:
Alloy Frame
120mm Lockout forks
Hydraulic Disc brakes
SRAM X5.0 27 Speed
Ofc you really should be looking around for the best deals, dont forget 2011 bikes are coming in soon meaning old 2010 stock will start to be majorly discounted so its the right time to pick up a bargain. You can use the Kraken spec to compare to other brands and find something similar for less money.
PS. Do not forget to go try some bikes out, the sizes vary brand to brand 20" in one might not feel like 20" in another.