From my experience, working as both a radar and tower controller within the london area, I have found the Police Helis to be very very flexible in how they do business. They won't just get in the way because they can and they are more than aware of the outcome of their decisions, but at the end of the day if the aircraft is Cat B then everyone else gets out of his/her way. (and before anyone says it; unless you are Cat A etc...)
P251/252 will only be non standard if they are on an actual shout, so cut them some slack- you never know what they are responding to.
MM