You are a policeman who has stopped a cyclist on this
wonderful bit of pavement alongside the busy A10 approaching the junction with the M11 that is clearly ideal for shared use - NOT. How far are you prepared to walk and in which direction to find out if it has a blue sign making it legal before you decide whether to make an arrest that may or may not be legal depending on the pavement status. While you are thinking about that, how long is it going to take you to have a look at the cyclist's lights to see if they have a kite mark on them?
Also you are a cyclist passing through and new to the area. What sort of checks should you do to make sure its legal before you ride on it? Do you just give up and ride on the A10 to be sure, spot other cyclists on it and hope they know the legality and are following it or walk up and down it to look for a sign before going back, getting on your bike and cycling on it?
I'm not aware of any maps or databases the police or cyclists could use that mark which pavements have been converted to shared use and which are still pavements to avoid them having to walk the pavement for hundreds of yards in some cases looking for a sign.