We occasionally get asked this by the bobbies when one lands in Liverpool. Last time I got the following info:
Having had yet another call from one of our control rooms, regarding a helicopter landing in the centre of Liverpool, I spoke to Bob Jones at Flight Ops to get the definitive answer.
A private pilot requires permission to land in a congested area for each occasion.
A commercial company can obtain permission for each occasion or obtain a “block booking” if it’s a regular occurrence.
For both, the landowner’s permission is reqd. They are required to inform the local Police, not to obtain their permission, but purely in case Mrs Jones calls to say that she thinks a helicopter’s just crashed. The Police have no say as to whether the aircraft can land or not.
Hope that's of help.