There are two Situations where the the P226 Mk 25 is wielded. The CQC Basics Situation has an SAS Recruit equipped with an L85A2 and a P226 Mk 25 as a sidearm. The Heavily Fortified Situation has the Operator Thatcher equipped with an AR33 and a P226 Mk 25.
This handgun is a decent, all-rounder secondary weapon. The P226 Mk 25 can quickly kill an enemy in a few carefully aimed shots, allowing versatility amongst all else.
The handgun's in-game designation, "P226 Mk 25" is technically inaccurate, both in usage and in spelling. Only the Navy SEALs would use designation Mk 25, and even then, it would be labeled as MK25. As it's used by the SAS, the appropriate designation would be L105A2, the British designation of the pistol.
An actual MK25 designation would also have a silver anchor that comes engraved on all MK25 pistols to represent the Navy SEALs, which this model lacks.