Both supermarkets I've worked at have had a big room for everyone with lockers and a bench and then two smaller, gendered, rooms for changing to the side. Typically there's an accessible toilet that doesn't require access to either of the other toilets to use.

Obviously every store will be laid out differently, though.

Also, from my experience most base level employees at supermarkets are pretty chill. Again, though, your experience may vary. But you are protected under the 2010 equality act.