NorthernDave
Never used Über Member
Depending how gnarly your off roading is likely to be, you might want to look at a hybrid or a CX bike instead?
An MTB will be slow and had work on road and unless you are really going for it, a hybrid or CX will cope with anything and be considerably nice to ride on road.
This is under your budget (and you'd get another 10% off if you join British Cycling)
http://www.halfords.com/cycling/bikes/hybrid-bikes/boardman-hybrid-comp-bike
An MTB will be slow and had work on road and unless you are really going for it, a hybrid or CX will cope with anything and be considerably nice to ride on road.
This is under your budget (and you'd get another 10% off if you join British Cycling)
http://www.halfords.com/cycling/bikes/hybrid-bikes/boardman-hybrid-comp-bike
albeit costs a bit more