Decide on budget, off the peg or custom, frame material etc. first, then it’ll make it easier to make a decision…
All the bikes you mention above are very good, I’d happily own any of them. I’ve got a Genesis Altitude, I think it’s a great bike, I can highly recommend one if that’s what you want me to say… But I could also highly recommend the Soul too, and I just love the look of those Whyte 19’s too!
Personally, I’m of the opinion that for the type of riding I prefer to do, steel is worth the 1-1.5lb weight penalty over ally. You need to decide that for yourself too really.
Size wise, at 5ft 8 you’re probably capable of riding a small or a Medium quite happily from most brands. If you’re really long in the leg and short in the torso, I’d suggest small, otherwise a medium is probably going to be a more efficient fit.
Anyway, when you planning on getting the bike? A quick internet search has revealed the following, in stock, in a medium size…
http://www.langsettcycles.co.uk/products.php?plid=m1b107s2p6811&rs=gb
And it’s a 2010 bike (in the nice cherry red colour) with £300 off! Much better to buy an outgoing model with a price reduction than a new bike with fancy stickers IMO.