oh, there used to be a @$500 MTB sticky, but it seems to have gotten lost.
£600 is a bit more than $500, but the same principles apply - if you want a MTB, go to a few bike shops and see what they have; you don't mention racing, so pay no attention to the entire weight issue (to a point, anyway). Look for fit & comfort. You can take a decent MTB & put slick tire on it for the road.
Or, you can get a cyclocross bike for road & field.... I'm thinking that you can get a pretty nice hardtail in your price range, but you have to simply go & see what's available & what you like.
As with mud, life, too, slides by.