Not a solicitor but....
High court declared, long ago, that any place into which the public are permitted access is a public place. (I think the case involved a RTA (YTA??) in which a car collided with a parked car in a gated co-op yard. The gates were kept permanently open, so the court decided that the area was public.
WRT using a mobile while driving...duh! Pay the fine and think yourself lucky that you are getting away lightly. If you had been in even the slightest tip, or caused one while using one, you would have been over a barrel.....and as the O'reilly case showed...big brother knows when you're on, so denial is futile.
....get a hands free or pull in......after all there was a hotel car park convenient to you!!!