I bought a Specialized Rockhopper from evanscycles, and first off, the company is excellent. Bike was delivered within 3 days, had a call off them when it was ready for dispatch, I was muchly chuffed with the service
With the Specialized Hardrock at £292, you're getting budget forks (suntours) and V-brakes, not discs. When I'm trailing, I like to know I've got a steady pair of discs that are going to stop me, especially if the trail happens to be a muddy one

but then you're looking at an extra ~£100ish.
The balance is that you're getting a quality frame, a quality bike in essence. Most of my riding is similar to yours, canal tracks, country roads, with moor trails included. I wouldn't recommend a hybrid, but that's just my preference. I like the feel of MB's and the comfort of knowing it'll cope if you want to 'upgrade' you riding at all.
The GT Avalanche
does have disc brakes however. I've got no experience with GT's (other than a quick test ride on one last year) but I'd say it's essential you try at least sitting on the bikes before you commit, if only to check the sizing is right
I'm 5'9" ish with a 31" leg; I ended up with a 15" frame, partly due to the shape of the Specialized, partly to do with the way I like to ride; you do need a fair chunk of clearance with a mountain bike as you don't want to be worrying that you'll get closely acquainted with the cross bar too often