No it isn't. A stop I am regularly at has several taps on their site which are marked as not being potable. They do have other taps which are fine to use for drinking water.All water coming through pipes is potable in the uk, the question is if its a very rarely used tap how long has that water been in that particular bit of pipework, your choice if you use it.
We have found morrisons petrol stations good for an outside tap and im led to believe all motorway service stations have too supply water somewhere on the forecourt.