Then definitely a lightweight hardtail. Full sus will be a big no no here…
You’ll probably want a full compliment of gears too, if you go for a suspension fork then no more than 100mm probably, and also a spare set of slick tyres will be useful.
Check out http://www.merlincycles.co.uk for their Rock Lobster frames, they can build you a full bike for a little over £1k with a very good spec. They’re very light, nimble frames that would suit your purposes very well.
Otherwise look at things like Cube’s Reaction, maybe Cannondale Flash’s, maybe a Giant XTC etc. But lightweight hardtails with lots of gears will be the key for you here, if you wanna ride on roads a fair bit as well as offroad.