Well, I'm not familiar with the rally you speak of, but after some research, it looks like you'll be riding about 54 miles with a kick-in-the-ass climb at the end. For commuting purposes as well, I would suggest the Surly Long Haul Trucker.

It costs much less than the 1000gbp you have (about $1000usd) but it's a wonderful commuter and tourer. It has drop handlebars in an upright-ish position for "long hauls" and every conceivable braze-on and attachment point for racks, fenders, water bottle cages etc. Stock, it runs wider touring tires for comfort and traction in all conditions, and you can have it in 700c or 26" wheel varieties. It has more of a road bike geometry (although relaxed and more upright) but it uses a Shimano XT mountain bike rear derailleur and cassette which gives you low gearing and big climbing ability.

In the UK, I imagine you'd be running fenders for the rain, and probably a rack to use commuting. This will fit the bill nicely, and with the money you saved, you could buy all the lights, accessories, clothing and attachments you need.