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.