Depends what you want to do, but don't touch anything for less than £800. If you read the magazines, 140 mm is the right amount of travel on the front. So a Specialized Stumpjumper would be viable. However, stumpys have a bad reliability record.
I would say a Trek Fuel EX8 These are just over 2K new, but have good kit and are relatively light weight for the 140mm (About 27lb). So this will see you around most trail centres and is a good all around machine.
The Ghost also came up as a good riding brand, so it is worth checking out their website.
If all you want to do is bomb over the local trails, with the occasional red run, then you probably don't need that much suspension, so 100 - 120 mm will do. I ride an Anthem, which has 100mm. Fast XC, but can get out of it's deapth on really big drops.
Like anything, it depends upon the style of riding you want to do. Good components to look out for are Fox (Forks and Shocks) or Rockshox. You also want either Shimano SLX or XT or SRAM X9 or X0. You get what you pay for.
The main thing is that, if you spend less than £800 on a full susser, then it is going to be heavy and your components will fail quickly. Do not buy any full susser for about £200. If this is your budget, there are better hard tails out there that will give you a better ride than a cheap susser.
Re-post your question with the style or riding you want to do and a rough budget and we can help you more.
Luck