I haven't ever encountered a situation in the US where I couldn't use the restroom in a store--at least not since the 80's (when I can remember grocery stores didn't have public restrooms). I think the Americans with Disabilities Act requires bathrooms for public use in most--if not all--stores. Lord knows in working with retail store leases I have seen a lot of mentions of building according to the ADA code and having a handicapped accessable bathroom.
The UK is much more relaxed in their laws about equal access for the disabled. But they are also frequently operating in buildings that are 100 years or more older and it can be difficult, if not impossible, to find the room for a lift, a bathroom or a wheelchair ramp. I know Kilkenny City in Ireland--dubbed the medieval city for good reason--was definitely not handicap friendly. But what you're going to do, tear up the "marble" sidewalks and try to widen them so someone can go down the way in a wheelchair? Unlike here, most of their buildings are old and historical.
I'll have to check on the ADA requirements and see if providing a public restroom is a requirement in the U.S. Could be good ammunition (if so) for people who encounter uncooperative shop owners.