"Restroom". Nothing quite so transatlantic, and so immediately American than for a Brit to hear the phrase "restroom".
There are all sorts of theories on the divergence of names for "the smallest room", most of which seem to agree on them being polite phrases to cover what happens therein. And although the French are probably right in that the British spend to much humour based on it, we do seem to have hijacked their "Toilette" for our usual polite name of the Toilet (the "William Crapper" notwithstanding).
Stack Exchange has a far too detailed discussion on what might breakdown the usage here https://english.stackexchange.com/questions/8281/washroom-restroom-bathroom-lavatory-toilet-or-toilet-room but in our simplistic view, it's more clear cut.
Restrooms in the UK are rare. It's not a subtle replacement of word or phrasing, unlike a Math/Maths type switch it's obvious, and out there, and makes a clear statement. And so companies like Subway who are actively using Restroom naming and signage in many of their restaurants (Slough Trading Estate, we're certainly looking at you) are to be commended (the cynical amongst us suspect this is cost savings to use the same signs globally rather than part of our grand plan, but we'll take it).
It might be some time before institutions like Marks and Spencer introduce the Restroom sign, but global franchises, frequented by the younger generation more likely to use the phrase, seems like the perfect place to start.