walk ups can be found in soho and a lesser known district of mayfair called shepherd market... but only just... soho is down to maybe a dozen and shepherd market a grand total of 2 (2 walk ups, 3 rooms / girls)...
when I started punting in 96 or 97 there loads of walk ups all over soho and shepherd market and my first punt was all the way over on pollen st by berkley square...
but the credit crunch, the change in punting habits (internet), aggressive land grabs and authoritarian oppression have reduced the scene to almost a memory and a part of the west ends history that seems to about to get conveniently swept under the carpet...
somebody once mentioned a walk up in kings cross but apart from that I have never heard about a walk up anywhere other than the west end (soho and shepherds market)...
on the other hand, if by "walk in" you meant somewhere you can just turn up then i'm sure there are still plenty of parlors around but they also seem to be in decline... its all online now isn't it.
but walk ups, as in a room with a door on the street and a sign openly inviting you to knock... no, just soho and shepherd market and they won't be there much longer either I don't think.