[IronDuke99]

The Royal Marines Fleet Protection Group.
This is a roughly battalion sized force.

It is responsible for the security of UK nuclear subs, bases and missiles. It also provides RM detachments for Royal Navy warships and Royal Fleet Auxiliary (supply and support) ships.

It is also trained to protect, and retake, oil rigs and merchant ships etc.

The whole unit (like the Special Forces Support Group and the Pathfinders of the Parachute Regt) uses the L119A2 Colt Canada 5.56mm Assault Rifle rather than the L85A2 SA80.

For scenario designers British warships (and RFA's) on overseas deployments normally carry at least a section of Royal Marines from the Fleet Protection Group for boarding and small scale landing operations and that number can be increased in 'high risk' areas like the Gulf.

As an aside UK is just now opening a permanent RN Base in Bahrain in the Gulf, the first new base East of Suez for many years.