Yea police boxes were already being phased out when the first episodes of Doctor Who aired as UK cops started getting radios. Don't think you'd have any chance of finding one now.

There are still a few of those red boxes in parts of the fairly big town I'm from but yea a lot were replaced with the black and stainless steel ones in the 1990s and I don't think any of the ones left standing have actual phones in them anymore.