Public potties on British Isles excursions

[cruisegal415]

We have excursions booked for all the ports on the Princess BI cruise next month and are spending 3 days in London pre-cruise. I know when we went to Europe we had to have euros available to use some public toilets. I assume the places we will be seeing have facilities in them, but doe we need to bring cash to use them? Trying to figure out how much extra to bring for bathrooms. 

[Cotswold Eagle]

As a general rule, expect to pay at (increasingly rare) standalone conveniences provided by the local council. Fee is often something like 20 p or 30 p. 
 

Those in visitor attractions, museums, galleries, large stores etc will be free, as will those in motorway service (rest) areas. Major railway stations now generally have free toilets too. 
 

Popping into a pub to use the loo without buying anything at the bar is frowned upon, but can be a last resort….

[Bob++]

In London, the best free facilities are to be found in department stores. They tend to not make them easy to find though, so you may need to ask for directions.