I don't know about websites but I can recommend a book called "Work on your accent", published by Collins. I use it with students.

Certainly NOT just 'any' TV series set in London! You'll hear all sorts of accents that way, and most of them are not RP. If you want some nice clear examples of RP in a TV series, try Downton Abbey. All of the Crawley family members (apart from the mother) speak in RP accents. If you're interested in accents, you'll hear the difference between the way that the family speak and the way that the servants speak.

You can search for this on youtube channels.

Hi Mattew, Thank you for your answer, could you please tell me some English TV series name?

You could see any series tv English (London) and learn from it.