Just been to Asda in Slough and in their MCDonalds restaraunt there is self service on soft drinks. I asked the man if it was free refills and he says it is. So you just get a small coke or whatever for 60 or 70p and its unlimited. Good deal I think.
UPDATE- Seems many stores are doing these refills please add in if you know a store near you which has free refills.