Police spotting (junction watch)

I'm not certain about Putney council having no authority, the city of London have some special rules about pavement cycling in certain areas (outside St. Paul's being the main one).

Councils can pass byelaws banning cycling from footways that are not part of the carriageway (as you say - Millenium Bridge, and other council owned land etc...) - Only the police can issue an FPN for the offence though.