Is it true that the police cannot make an arrest on Crown property such as a Jobcentre or a tax office? I witnessed a middle management civil servant telling a police officer that he could not arrest a chap there without prior arrangement. He got away.
I worked in a Benefit office and people were arrested, but usually the police took them outside to arrest so that they hadn't got an audience to act up to, and also they hadn't got 'friends' trying to stop the arrest.
And anyway, the concept of Crown property doesn't exist anymore (except perhaps for that owned by the Queen). Government department buildings are very often leased by the Government department involved - the land is actually owned by a property company, having been the subject of sale / leaseback deals..