My understanding is that harbours which accomodate visiting yachts, fishing boats etc have CDPs and water points (and showers), but I think you have to pay a fee to the Harbour Master. There's a famous toilet block on Mull at beautiful Calgary Bay (wonder if it's still there? it was famous for being so well looked after) - it had an outside tap, and every time I passed there was a MH with a very long hose parked up beside it, filling up. As for fuel stations, Sainsbury does not charge for its air or water.
I wouldn't have thought obtaining water would be too much of a problem providing you ask at garages before filling up and don't just take it for granted they won't mind.
I also once saw a program on the TV about natural spring water and it showed a man filling up loads of containers from a well in the middle of a town...Leamington Spa or Bath might be good places to visit
I would imagine the difficult bit would be getting rid of the waste water as there is only so much you can store in jerry cans. Having said that, pouring waste water down a drain in a layby might not be a problem. Given the noxious liquids that I am sure must get poured down there from time to time a few gallons of soapy water might do it some good.
There's a free water tap in the centre of Buxton too, there's always a number of people there filling bottles from their car boot.
If you are travelling and working or visiting, just how much water do you need? Use supermarket toilets as much as possible, and as suggested, use wet wipes for body cleansing. Or visit a swimming pool and have a shower and hairwash there. Eat takeaways, so little washing up, or use disposible plates and cutlery. Or you can bag up the cutlery and wash it when you get home.
I remember when showers were a rarity years ago you could buy a rubber tube that attached to the bath taps. Could you not adapt a garden hose clip to connect to a tap in a public toilet. Keep a separate supply for personal consumption though.