British term for 'washroom'? [duplicate]

Solution 1:

The equivalent would be the word toilet. That is used generally. If the context means that someone is referring to a place that has more than one toilet, for example a school; the word to use would be toilets.

Solution 2:

Warning: Social Minefield!

Lavatory, loo and toilet will all be understood, which is in many cases the only real desideratum. However, which word to use (as well as proper employment of slang like bog and khazi, which though informal are far from entirely proletarian) will have a major impact on how you are perceived, and the etiquette is far too complex to summarize in less than a monograph. Frank Muir, in A Book at Bathtime, observed that where "Would you like to wash your hands?" is likely to be misunderstood, a well-brought-up host will ask "Would you like to...?" (minimal gesture) and wait for the answer. However long it takes.