This is a list of all UK police forces and what we believe their force radio callsign to be.
We say 'believe' because of course they don't generally publicize this information.
According to various sources all UK police forces have a callsign starting with 'M2'. This fits into international callsign allocations where the UK is allocated all callsigns starting with G and M. The 'M2' is followed by two letters.
 
For day to day intra-force communication purposes it appears that only the last two letters are spoken usually as - for example 'AB' - or as the phonetic codes for the letters ('Alpha Bravo'); the 'M2' is not spoken, but we believe it is used when contacting other agencies, for example the fire service, mountain rescue teams, or other police forces. As it is an international callsign it could potentially be used for communicating with foreign agencies too (e.g. across the English Channel, and to the Irish Police).
 
Some callsigns seem pretty certain because they tie in with those used in various TV programmes, from “BD to Z Victor 1” in Z Cars back in the sixties, to ‘MP’ in The Bill; and some have been heard on various police reality shows. Those familiar with the classic 1955 Ealing comedy The Ladykillers, may recall the use of ‘M2GW’, which was presumably the forerunner to ‘M2MP’. The same callsign was also used in the 1951 film The Lavender Hill Mob.
 
What use is this information?
Ah well, each force has a unique four character callsign which is effectively an 'official' abbreviation for the force, and a very handy Twitter tag — it's much easier and shorter to write ‘#M2MP’ than ‘#MetropolitanPolice’.
 
 
ForceCallsign & Twitter tag search link (new tab)
Avon and SomersetM2QP
BedfordshireM2VA
CambridgeshireM2VB
Central Police (Stirling, Falkirk, Clackmannanshire)M2AH
CheshireM2BA
City of LondonM2CP ?
ClevelandM2LZ
CMPG¹ (West Mids, Staffs, West Mercia, Warks motorway patrols)M2MA
CumbriaM2BB
DerbyshireM2NA
Devon and CornwallM2QB
DorsetM2QC
Dumfries and GallowayM2AJ
DurhamM2LA
Dyfed PowysM2WH
EssexM2VG
FifeM2ZT
GloucestershireM2QL
Grampian (Aberdeenshire, Moray)M2UB
Greater ManchesterM2CK
GuernseyM2GY
GwentM2WE
HampshireM2HC
HertfordshireM2VH
HumbersideM2XH
Isle of ManM2MX
Isle of Wight (Hampshire Constabulary)M2HW
JerseyM2GS
KentM2KA
LancashireM2BD
LeicestershireM2NL
LincolnshireM2NC
Lothian and BordersM2ZH
MerseysideM2CH
MetropolitanM2MP
NorfolkM2VK
North WalesM2WA
North YorkshireM2XN
NorthamptonshireM2NG
Northern Constabulary (Highlands & Islands)M2UR
NorthumberlandM2LB
NottinghamshireM2NH
PSNI (Northern Ireland)Unknown, we'll just use PSNI
South WalesM2WL
South YorkshireM2XS
StaffordshireM2YF
StrathclydeM2AS
SuffolkM2VL
SurreyM2HJ
SussexM2KB
Tayside (Perth & Kinross, Angus, Dundee)M2ZS
Thames Valley² - BerkshireM2HU
Thames Valley² - BuckinghamshireM2HB
Thames Valley² - OxfordshireM2HA
WarwickshireM2YJ
West Mercia (Herefordshire, Worcestershire, Shropshire)M2YK
West MidlandsM2YM
West YorkshireM2XW
WiltshireM2QJ

The information above has been compiled from various sources originating from several years. We cannot vouch for its accuracy or contemporaneity, but these are the callsigns that we may use as abbreviations, especially to tag Twitter posts.

1. CMPG (Central Motorway Police Group) is a multi-force agency responsible for policing motorways in West Midlands, Staffordshire, West Mercia, and Warkwickshire. They use the phonetic code callsign ‘Mike Alpha’ (M2MA) - this has been heard numerous times on TV documentaries about motorway police. There is said to be a similar group for the North West.

2. We don't know why Thames Valley Police seem to have three callsigns when every other force has one. Perhaps it is just historical from before TVP came into existence. We'll use 'M2HA' when refering to TVP because it comes first alphabetically.

3. The above callsigns may be partially out of date. When fire brigades began using the new TETRA digital radios in 2007/8/9 they changed all their callsigns to facilitate communication with other emergency services. It is possible the police did likewise, or maybe they just replaced ‘M2’ by ‘P’. This photo from from Oct 2010 clearly shows two letter callsigns on police riot helmets (NL=Leics, NH=Notts, CK=GMP), indicating that at least the letters are still in use.
 
References
Home Office Radio
Police on Medium Wave
See also our guide to UK Police on Twitter