I used to have Old English Mastiffs -- the biggest one was 180 pounds -- and they're great dogs, but kind of wimpy on the trail. They're so big and powerful that they wear out easily I was never, ever, able to get one of them to go more than 10 miles in a day, usually less.
Plus they require a TON of food. All the same, I love 'em.
Currently I have a cockapoo (cocker spaniel and poodle mix, horrible name, though "cockapoo..."). At about 24 pounds, he's easy to deal with and boy, all poodle mixes I've run into are absurdly smart and woodsy. Unflappable. Because he comes from hunting breeds, he's a helluva good bush dog and sweet of temperament.
However, I find myself fascinated by mountain curs --
http://en.wikipedia.org/wiki/Mountain_Cur -- which are legendary bush dogs. They are said to possess all the perfect attributes you want in wild places: they are THE dogs of the American frontier.
(That said, Lewis took his Newfoundland on the Lewis and Clark expedition and the beast made it over and back. Even though they ate a lot of dogs, they loved that Newfie too much to eat...)
Mountain Curs can be pretty hard to find in the US and I imagine well nigh impossible to find in the UK. But one of these days, I think I might have to get one...