Actually, I've had a word with my wife, and she wouldn't recommend the Asda car park wash now after all. In recent times, they seem to have got more people involved to rotate the work, and the standard seems to be rather hit and miss, perhaps dependant on who you get doing it.
It's £20 for...
You could try Hollingbury Asda - that's my wife's preferred option. A team in the car park will clean your car inside and out while you shop. It's always ready by the time she finishes her shop.