Have a look at a Specialized Hardrock - all the range of 'rocks should fit your budget. I've got one of the older incarnations, but it did me well for years. It was all the MTB I ever needed (until I bust my knee

), with a readily upgradeable frame - and from what I understand the current range is better than ever.
There's a
Decathlon in Sheffield isn't there? Some of their Rockrider MTBs get very good reviews for the price.