Hmm
What about:-
http://www.decathlon.co.uk/EN/rockrider-5-2-black-34963795/
http://www.decathlon.co.uk/EN/rockrider-5-3-69568390/
http://www.decathlon.co.uk/EN/rockrider-5-xc-disc-34964379/
Here you have a £200 bike as good as some of your options, a £299 bike better in many ways and a £369 bike which is MUCH BETTER even though it goes slightly over your limit.
Basically, you have missed out by a week on the Aspire 1.0 which was shedloads better than the 2.0 and only £35 more, it was an amazing deal with Toras and 27 speed Deore groupset for £275 which is about double the spec of the 10% cheaper 1.0. I was planning to get one myself as a spare and missed out as well.
Personally, I dont know how good Dart 1's are, but the Aspire 1.0 seems to have some really weak parts on it compared to the 2.0, like an ugly, cheesy, plastic coated mild steel Tourney chainset and Altus shifters. These parts are right at the very bottom of Shimano's groupsets. I also have serious doubts about Dart 1's expecting them only to be about as good as Insync Grind or cheap Suntour jobbies but with the Rock Shox cred branding.
The £369
Decathlon blows away the Focus, its slightly lighter, it has a MUCH better, higher end fork from the same company (Suntour), hydraulic disc brakes (which are superior to mechanical discs or v brakes), better (disc ready) wheels than the Focus, etc, etc.
The Focus is otherwise about the same level as the Decathlon, but with cheaper wheels, brakes and forks.
I have a Vulcan V spec (although I fitted £200 Magura Odurs to it), its quite a good bike for under £250 and lasted a couple of thousand miles before the parts started to wear out.
I would find the extra £20 and buy the Decathlon, you may need to ring them and ask them to order one if the nearest store to you does not have the one you want in your size in stock. If you just want a good, cheap bike for one occassion the £199 Decathlon is about as good as the Vulcan or the Aspire and cheaper.
Secondly I would go for the Aspire 2.0 assuming that Darts SHOULD be better than the cheap Suntour forks on the others and because it has £160 off but I am still dubious about the Tourney and Altus bits.
Thirdly the Carrera Vulcan is a good value bike if you go for the V spec. Ignore the Disc spec, they are only crappy mechanical tektro's (which will prove more hassle in the long run) for an extra £100 and the rest of the bike is the same and I cannot see why anyone would want to pay £100 extra for mechanical discs over V's.
I dont know the GT but the Focus has some crap parts at that price, especially when you take the supposed full price into account, unlike the Focus Black Forest which at £700 is basically a bike with £600 forks!