My point is that the OP himself says he should have known better than to buy a Halfords bike. I suspect that is down to the fact that many, many posts come from the start point that Halfords bikes are crap. They aren't. People are quick to condemn Halfords out of hand, which is grossly unfair and not helpful to a novice. Their bikes, at least their high end stuff, are excellent value for money in terms of componentry and bang for buck. He has bought into that and then excuses his choice of bike by saying he was constrained by the C2W.
Posts tell him then that Halfords bikes are OK as long as you don't let Halfords spanner monkeys anywhere near them. Again, grossly unfair, I know two of the local Halfords mechanics and their knowledge and skill are right up there with LBS staff. In some cases much better.
The usual suspects then say "get someone competent to build it. " which is fine, great advice you'd think, but here's where the OPmcomes unstuck. He sees the bike in a box and wonders just how hard can this be, but now we know it isn't that easy to get right. Personal pride and a bit of macho, and man the provider and doer swings into play. He then chooses to ignore the last bit of the advice which is most crucial.
From here it goes all wrong. We don't know why the damage happened, but the OP is convinced it's down to poor quality components. The thing is, without an expert opinion, it could be several things, but it is not an everyday sort of catastrophe. Mechs and mech hangers don't go hurling themselves into the spokes or wrapping round the frame on their own. From Halfords point of view, the "you built it, it's your problem" is an obvious answer. They have offered to split the bill, quite generously. Talk of taking them to small claims are frankly ridiculous.
I'm no great fan of Halfords by the way. And have seen poor build examples myself, but if the forumites had one good piece of advice, it's lost in a sea of Halfords are shite type threads.