New 20 MPH speed limit to also apply to bikes in Southwark

True - but the police do use them - rated and approved for use on bicycles or not.
Has a cyclist ever been prosecuted or are they just used to 'warn'?

FPNs are issued. One person (that I have read about) was taken to court.

Because there have been know challenges (as far as I am aware of) - I.e. everyone pays the FPN, and the chap that went to court pleaded guilty - they are used as bluster.