661, there are usualy last years colours cheep on CRC.
I got a strike for £35 and very happy with it for BMX etc.
Fit is the most important thing, but I'd question whether a halfords full face would meet the same criteria as a branded one, or if its just the same protection as an XC lid and the rest is just for show.
Try a few on, you can pick up mid/high end helmets in last years colours for <£60 and will be better designed than a halfords one, either more protecting, better ventilated or better looking depending on your priority. And the price difference is less than a night in the pub (a double whamy, better protected brain cells, and less dead from alcohol poisoning :p )