'a couple of quid'. Ho bloody ho. It's quite incredible how this country whose wealth has been built on engineering now values engineering skills so little.
The chance of a general engineering shop have a bike specific helicoil is very remote indeed, effectively zero. The engineering company may be persuaded to buy a helicoil kit, they cost about £30.00 but he won't be able to source one from his usual tool supplier, so he'll have to spend some time shoping around. When he's bought it he'll probably take a 1/2 hour making the repair. Charging £35 an hour, probably more.
So your 'couple of quid' will likely to be a more realistic £45+, but you'll be able to keep the really useful helicoil kit.
BTW Highpath Engineering offer a repair service, £20.00 per crank plus postage. But not surprisingly they will already have the repair kit.
http://www.highpath.net/cycles/special/repairs.html
Last time I needed on repairing I tried a very reputable bike shop in Oxford, Walton Street Cycles. Yes they could do it, but they'd run out of inserts and had lost the special taps. Useless. My guess is you'll struggle to find a bike shop to do it. Mine went to Highpath who did an excellent job at a very reasonable £20.00.
BTW I run a small engineering company. I have most metric helicoil kits (up to M16) to hand, but still thought it better value to send it to Highpath.