Nope - they are allowed to use ANPR and do and use some very unsavoury companies to enforce it. Some people will defend it's use at hospitals but IMO it's nothing more than a tax on the sick. Parking should be free as it is in Wales and Scotland.
I don't have an issue with a council issuing a ticket and getting it paid ( just as long as it's valid ) BUT I do have an issue with a private company doing it - especially as these private companies use bullying and in many cases criminal practices to get the money. There is no real control...
They have to give you a reasonable amount of time to decide if you wish to stay - i.e. agree to the T&Cs. Normally it's about 10 minutes but the OP was pushing it with 20 minutes. That said it's run by scammers / criminals so needs fighting as LadySeagull is going to do for the OP.