Legality of opening someone else's car door?
If you opened someone's car door you could be committing either Dangerous cycling or Careless, and inconsiderate, cycling. (Road Traffic Act 1988)

There's also the offence of Assault to consider. (CPS guidelines)


  An assault is committed when a person intentionally or recklessly causes another to apprehend the immediate infliction of unlawful force.


Aside from the legality question, it's probably worth pointing out that many cars automatically lock the doors once the car starts moving and don't unlock them until the engine is switched off.