All the above are great places to shop, I have frequented all of them. For medium and large format, I'd also recommend Teamwork Photo. And if you're interested in Leica at all, The Classic Camera is right next to the British Museum; bit sad that Richard Caplan closed down several months back.
Finally, this is a bit off the beaten track, but you *should* try to get down to Croydon and check out High Street Radio and Photographic (
http://www.croydonphotocentre.co.uk). The proprietor, Reg Roach, is 89 years old, but he's been selling Leica and lots of other second hand film gear in the same spot for over 60 years. He's a legend, and a treasure--stunning knowledge of all things photographic, on the film front certainly. Make sure you check their hours, because due to the pandemic they're quite limited. And you might want to call ahead to make sure Reg is in the shop that day, his son Paul runs it in his absence--but a chat with Reg is definitely worth the trip down to South London.