Willow, did you read tha PDF that you posted the link to (I posted the same link some time ago in another thread btw!
) Basically when on Private property they can restrict you in any way they want. If that is to make you 'buy' an official piccy then so be it.
The other thing to be wary of is that the link relates to teh law as at August 2004. Some time ago in this day and age so i wouldn't take it as total gospel without checking if there are any new developments or laws or changes to existing legislation e.g. data protection act