Hi,
I'm going to guess that you are in Edinburgh, although I know there are several CO-OP shops around the country. If you are I know at least some of the trails you are going to be riding, Glentress, Carron Valley etc. I would personally go with a 26"/27.5" wheel, 26" is what these trails were designed for and if you are just getting into mountain biking you may struggle on some of the tight switchbacks with a 29er. In fact, most of the UK trails were designed for 26" and that size still dominates, they are a bit different to the long trails you get in the US where mile munching can also come into play. Yes, 29ers will roll better over rock-gardens etc. but I personally think you'll have more up on a 26" wheel. I'm 6 foot and ride a large frame (19") on 26" wheel and it works very well. You'll also learn better skills on a hardtail 26" bike.
I don't want to push up your budget, as you'll also need to buy some decent pedals, shoes, probably some shorts at least and maybe some other things. I'll suggest you look at this:
http://www.edinburghbicycle.com/products/genesis-core-10-14
Same as the Revolution price wise and the geometry will suit the kind of thing you will be riding, plus the 27.5 wheels are going to push 26" out of the market, 29" hasn't taken as big a hold in the UK as it has in the US, so you won't be changing wheel size on future bikes.
Any of those bikes will get you round UK trails though unless you point it down a downhill run at Fort William.
A quick word on upgrading. Yes, upgrading is a great route, it's what I've done, but bear in mind all of these frames are probably rated to 120mm travel forks at the most and are 1 1/8th steerer tube only, these kinds of forks are getting rarer and you are more likely to either upgrade to a new bike or a new frame, which is what I've just done along with the fork.
Main thing is don't over-think it, any of those bikes will get you round the trails fine, will last a good few years and can be upgraded if that's what you choose to do. I'd say go with your heart if your head is getting too fuzzy.
If you are around Edinburgh and want someone to show you round some of the trail centres give me a shout, the good weather we've had over the last weekend has been brilliant for mountain biking. I just built up a new frame with bits from my old bike, here it is on my shakedown ride at Carron:
Not a bad place to stop and tune up your gears...