Definitely, definitely get that Vulcan. Print out the online price and they will maybe match it instore (I presume it's order online for instore build and pickup anyway)
I wouldn't rule out even the 18" - I have 34" inside leg and would take a 18" carrera, they have comparatively long top tubes I think, but then you are taller than me (just have shorter legs) so who knows? You definitely need to try them out - I should think the 22" is definitely too big though. Vulcan/Kraken/Fury frames are all different but I think they have the same geometry so you can size up on any one in the range. And, anecdotally halfords are very good about exchanging for size reasons anyway.
Sizing also depends what you mean by 'light off road'. I'd say go for at least 2" - 3" standover height if you go offroad at all, then think about how much stretch you want. More stretched out (to a point) is more efficient pedalling but until you are an expert cross-country racer you may well prefer a more upright position. For your commuting, you can cope with a too-big bike, it won't matter as much. But I would say err on the side of smallness - you can always get a longer stem and layback seatpost for a bike that is too small but there is nothing to help with crushing your knackers on a frame that is too big.