Not sure which you local Halfords is, but Boardman's cost from less than £400, with the parts you have listed, you will be pushing this, and still have a very basic frame/other components.

For the gear ratios, are you carrying anything on the bike, or is it just you, if just you, would could get away with a 32T at the back and a compact up front, if carrying, then a triple would be better, having ridden N.Wales & Snowdonia a bit this year, on a compact & 11-32 worth approx 7kg on the back, it was all doable, having previously looked at gear ratios, there was very little difference in the 34/32 to a triple 30/28 that it wasn't worth the cost of changing.