Climbing On or Jumping From Buildings is a crime under Section 8A of the Summary Offences Act 1988 which carries a maximum penalty of 3 months in prison.
To establish the offence, the prosecution must prove beyond reasonable doubt that:
A ‘structure’ includes a bridge, crane or tower but does not include a structure designated for recreational climbing or jumping.
A defence to the charge is that you had ‘some reason or excuse’ for doing the act, or you did it for a ‘lawful purpose’.