As far as I'm aware, generic non-pressure spare parts for sporting rifles are not covered by the ITAR restrictions in the US. I have not had any problems purchasing magazines for my Rugers in the US and bringing them back through US and UK security. Your main problem with mail order will be finding a retailer willing to ship ANY firearm related item to the UK full stop - they are terrified of putting a foot wrong.
Bits like magazines aren't covered by your FAC and don't need the involvement of an RFD - we get a mate's Czech' housekeeper to buy CZ magazines from the factory outlet whenever she goes home to Brno, as they're half the price that Edgar Bros charge here in the UK.
I understand the rules are different for parts for millitary derived AR15 style rifles - I'm currently at the GAO Show in Harrisburg, Pennsylvania and am trying to clarify if I can bring back hi-capacity .223 AR15 polymer magazines for a mate, as the ones he wants cost me £7 trade here as opposed to £30 in the UK.
Adam.
Edit: If you can get it shipped to my hotel in Harrisburg before Monday the 10th February, I'll bring it back for you and post it in the UK. Drop me a PM.