I've had two wheels repaired by Pristine Wheels, based in Woburn Sands near Milton Keynes. I'd recommend them unreservedly - they will collect the damaged wheel with the tyre still on, and return it, as new, colour matched, rebalanced, new valve etc in around 4 working days, including transit time to and from you. They can even weld new metal onto the wheel rim if you've gouged them really badly, and they completely repaint and relacquer the wheel, which I don't think the mobile vans can do. I did once call out a mobile van to repair a wheel on my previous car, and they did only charge about 40 quid, but they really didn't do a good job - basically just sanded down and resprayed half the wheel with not quite the right colour of silver paint, and I had to relacquer it myself, which I wouldn't recommend.
Pristine aren't cheap - I paid £80-90 per wheel (depends on size of wheel and severity of damage), including shipping and VAT, but you can't tell the repaired wheels from brand new ones, and it's a damn sight cheaper than having to buy new wheels at £300 a corner...