I think you should get a mountain bike. I got one last year after being a committed road bike person for years. As I live on the doorstep of the Pentland Hills there are tonnes of good tracks within ten minute’s ride, so it seemed silly not to. Mountain biking is a different discipline and I find the style different from road-biking but it’s bloody good fun.
As my starter bike I went for a Specialized Hardrock – it’s not particularly rugged and as a result I keep breaking it but it was relatively lightweight (though about three tonnes heavier than my road bike!) and relatively cheap. If I was buying again, I would probably save up a few more hundred pounds and go for something tougher.
* this isn’t true by the way.