I’m not sure if they do developing, but I’ve bought film (and developing stuff) from Parallax in Brixton and they’ve been decent.
Yeah Parallax is a good shop to buy film and kit - I've bought chemicals from there. I don't think they do actual development but they might know where you can get it done in Brixton.
The shop under the Photographers' Gallery is probably the best physical shop in central London. Lots of rare and experimental films as well as the normal types. I always browse that afterwards if I visit the gallery.
Mostly I buy film online tbh, generally from
Analogue Wonderland | Buy Film Online | Develop Your Film | 35mm, 120 who do develop as well (not used that) but I don't think print, they'll just do scans, which you could print from of course.
I usually post my colour film off for development, I use
Home - Ag Photographic+Photolab and they definitely do prints. Almost any Snappy Snaps will do colour dev/print/scan in the shop, but the quality can vary... I know the one on Wardour Street is good (they still get media work apparently) and there's one in Covent Garden I've had good results from too. There are some indie places in the East End but I don't really know who is still going any more. I don't know who will do B&W since I dev that myself but it's usually a little pricier as they can't use minilabs.
Colour film is expensive in general these days btw. Kodak put their prices up and there were issues with supply; Fuji have cut their stock drastically and it never seems to be available; nobody else really makes the stuff. I have a load of colour in my drawer that I never shot over lockdown and am gradually working through, but I don't plan to buy any more once that's gone, except in Polaroid... HP5+ does the job for me.