In London Metropolitan Police area the radio controller is always referred to as MP. Does this mean the same as the force or is it used elsewhere in the country by all controllers and means something else?

Different over the country. Lancashire is BD, Cheshire is CH. Possibly connected in some cases with the force name, MP and CH would be examples of this, but some obviously not as it's very hard to connect BD to Lancashire. The Lancashire headquarters is at Hutton, near Preston, which doesn't help either. So far as I can remember, the mobiles are technically prefixed with the base sign as well. (This first came to a wider public awareness with 'Z Cars' which constantly referred to BD.) Different organisations have special call signs as well. St John Ambulance has regionally coded call signs, my local area being NE mwith the following number being coded to indicate type of user.

Useful to get the national picture, seems more like American radio stations who are allocated callsigns, some which mean something, others in sequence. Watch for my next question which I've never had a squeak about from anywhere!
It was 67306, see if you can have a crack at it.