I have found that there are alot more pumps that take UK cards around the main tourist areas (I drive, 192 etc), so you don't have to worry about going in to pre-pay. Those further away from the main areas tend to be pre-pay. I have not found it a problem, just the usual panic when you first fill up trying to work out how much you need, but as noted before, if you paid too much, they will just credit back on your card.