The loo

Yes, you can still pop in.

Did that in July at the National Gallery. Have also used loos at Costa Coffees, Pret a Mangers and the Red Lion Pub on Whitehall. Its up a steep flight of stairs.

There are a number of museums that don't need a timed ticket where you can just pop in and use the restroom. I was just in London last week and never had a problem finding a restroom when I needed one.

Many of the London museums are free, which definitely makes their WCs worth considering. However, there may be considerable walking involved to get to the WC even after you reach the front door of the museum, so this may not be the fastest option unless you were already planning for the museum to be your next sightseeing stop.

It's not unheard-of for chain-restaurant WCs to have numeric keypad locks; you get the code on your receipt after buying something. I think it was a Pret a Manger where I ran into that.

Museums and department stores (not that there are many of those left) - available and free. Probably tucked away on the highest floor or adjacent to the cafe.

Cafes and restaurants provide a service for their customers, so it’s rude to use their facilities if you aren’t buying anything. The McDonalds near Charing Cross station needs a code when I was last in there (quite some time ago). Ditto hotel lobbies - the facilities won’t always be obvious and maybe behind a door where you need an access key.

Rail stations - available but you need to pay.

Bigger supermarkets are free, but you won’t find these in central London. The smaller stores don’t have WCs.

The rail stations are good to remember. All stations in a city of reasonable size will have WCs. In the large stations, signage will point you in the right direction. There is, unfortunately, a slight possibility the facility will be closed for cleaning when you arrive.

Transport for London (TfL) publishes many excellent maps regarding transport in London, including https://tfl.gov.uk/help-and-contact/public-toilets-in-london which not only provides details of facilities on TfL properties but also nearby and with links to other sources throughout London.

Rail stations are generally a safe bet. Be advised, though, you may need some small change to use them.

Many museums in London are free and will have toilets. Just remember that the loos may not necessarily be located at the entrance to the museum.

Cafes/coffee shops - my experience of late has been that many are putting locks on the doors and you need to get a code from the till when you purchase something. Not universal, but something to be aware of. Have had good luck with Nero and Costa. Much less luck with Starbucks.

Most grocery stores in London are small and will not have public toilets.

Department stores - these generally have facilities, but remember that they may be located on the top floor and in a secluded/out of the way corner.

Most hotels will have facilities just off the main lobby, though they may not be obvious.

Pubs will invariably have facilities - although they may be up or down a set of steep stairs. Just be mindful that some pubs in busy areas (Covent Garden, for example) may have signage that indicates the loos are for customers only.

Yes, London one of the easiest cities where you can pop in to poop.

Sorry, I just had to.....write it.......not the other thing.

This is a very needed question and I was eventually going to ask it but you did so.

I am relieved (no pun intended) to see that it is easy to find loo's in London. I was told differently when I asked about toilets in Rome.