In message , James
writes

I read some previous threads on the speed cams at Tower Bridge, but they
didn't quite answer a query I have, I wondered if any of the experts on
these groups might no. I received a NIP (notice of intended prosecution)
not long ago for doing 32mph and it defines the speed limit there as 20. I
did to be fair see these signs, but always understood that 30 was the legal
minimum speed limit in the UK, and that therefore signs indicating 20 were
advisory. Is this still true?

If it ever has been, which I doubt, it is certainly not the case now. If
the signs were legal, then 20 is the limit.

There are many, many places where 20mph is now the limit if signed -
outside schools, in various inner-town areas, and in the whole of
certain defined areas such as the roads within Richmond Park.

Welcome to the new world - swallow hard and accept it, unless you can
afford a lawyer who is able successfully to argue that the signs were
not legal or the camera faulty. Since the Tower Bridge limit has
apparently been there for half a century, I think it unlikely:

http://www.speedcheck.co.uk/pressStory19.htm

--
Paul Terry