What do you want it for?  If its commuting then dirt bike/downhill/jump bike style etc are all a little overkill.  The fat tyres will just make it hard work.  

If you really want an off road bike then Id probably pick up a second hand bike for that money.  Something like a Specialized Hardrock (or equivalent from Giant, think its the XTC model of memory serves).  Something local to you but along these lines would be good:
http://cgi.ebay.co.uk/specialized-hardrock-sport-19-mountain-bike_W0QQitemZ150217074374QQihZ005QQcategoryZ33503QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

For an actual road bike (which it sounds like what youd actually be best with) I wouldnt have a clue.


edit: Heres some recommendations at about your budget from a fairly knowing crowd from a bike forum I use.
http://www.cyclechat.co.uk/forums/showthread.php?t=7013

a £150 bike wont last 2 minutes in DH/DJ and jumping.

Save up and buy something 5x as expensive if you want it to last, handle or actually work.
Anything sub £200 isnt capable off road.

Barracudas are gash.  Youd be better scouring ebay and getting bits second hand.  Not worth getting a second hand frame unless you know its history.
The amount of ebay frames we saw at Stif that were cracked was unbelievable.

Bit of both.
Your not going to get a good reliable bike that can take abuse for less than £300 tbh.
If your after a commuter that no-ones going to think twice about then just get some poc from ebay and save and get a bike for best.

My fiances learnt this the hard way having her Kona Caldera stolen from uni recently.

Ye think thats a gud idea, il keep my meccy for now, and then see how it goes, keep me fitter sloggin up the hill on a bmx, only cost me £20 but its worth £180 brand new lol think i will save up tho cz wud be nice to get back into offroadin.think when i do i think gona spend bout £200- £260 n get something from the kona range 2nd hand. like dirt jump bikes cz they gud for free style, like i said im ok at trials so nice to have sumut lightweight i can throw about, bt at same time have gears so my legs aint rotatin like road-runner. cheers for the advice guys.

Bit of both.
Your not going to get a good reliable bike that can take abuse for less than £300 tbh.
If your after a commuter that no-ones going to think twice about then just get some poc from ebay and save and get a bike for best.

My fiances learnt this the hard way having her Kona Caldera stolen from uni recently.