Must have been at or near the Embankment station. We just walked across the Thames (foot bridge) and had dinner and used theirs.
Toursist guide noted “just walk into restaurants like you own the place and use their facilities”.
London and Paris were weird, almost ZERO public restrooms/water-closets unless you pay a pound or euro. Oh, but in the Paris metro you could smell those who used it for free... many parts (from the Arc de Triomphe area) smelled of it badly.... wow, Paris stunk, in general.
There’s an Android app for that: “Toilets in Paris”. Seriously!