Maybe something like the .280/30 that us Brits put forward many many moons ago but were forced to drop it in favour of the 7.62mm x 51 by them damn Yankees who once every one in NATO adopted the 7.62mm changed to 5.56mm.
We also appear to be after a new CQB bundle of joy as published on the EDA website:
Expressions of interest are sought from companies wishing to be considered for the provision of qty 300 5.56mm Close Quarter Lethality. The weapon system comprises the weapons and associated ancilliaries. A Modified OFF The Shelf (MOTS) procurement is envisaged. the following Key User Requirements (KURs) must be met by any system proposed for this requirement. The KURs are:
1) The user requires a rifle capable of delivering an incapacitating effect in an unarmoured human target at 200m range.
2) The user shall be provided with a rifle that can be brought to bear in confined spaces, overall length (with butt folded or retracted) no more thna 560mm with an objective overall length of 500mm.
3) The user shall be provided with a rifle with a folding or extendable butt.
4) The user shall be provided with a rifle that can be fired and operated from either the right or left shoulder.
5) The user shall be provided with a rifle that can fit the in-service SureFire flash hider via a threaded barrel end.
6) The user shall be provided with a removable NVH-compatible, 4MOA mini red dot/holographic CQB sight, fitted via a Mil Std 1913 Picatinny or STANAG 4694 NATO Accessory Rail interface.
7) The user shall be provided with a rifle capable of firing all UK in-service 5.56 x 45mm ammunition natures.
The user shall be provided with a rifle that is not degraded by prolonged exposure to a salt fog environment.
9) The user requires that the weapon is Safe and Suitable for Service (S3).
100 The user requires that the weapon must not contravene any aspect of the Geneva Convention, Hague Protocol or International Humanitarian Law.
11) The user requires that the weapon is safe in storage, transport, use, maintenance and disposal.
12) The user must be able to fire the system in the following operating environments as defined in DEFSTAN 00-35 (A1-3, B1-3, C1-2, M1-2).
CP teams are predominantly RMP, with a few trained RM Police Troop and RAF Police Flight. Occasionally they are suplimented by RLC PMS drivers. At the moment it is a 'closed' course and not open to other Arms and Branches. I believe the PMS drivers used to use MP5s, until the introduction of the L22 Carbine, where there is now no need to issue a smaller weapon as they can come equipt and already trained on their own weapon system.