Hi there, the UK does not currently field the Mk.23 SOCOM pistol, as far as I am aware it is only used by US Special Forces. Both the British and US Special Operations class used to have the SIG P226N, but since ingame SF have switched over to silenced pistols they have changed over to the M9. This may be changed in future releases, but currently as it is a broadly asthetic change is considered low prioity.
The British sniper currently has the L9A1, which is the standard issue British army sidearm.
While I don't recall the Mk.23 for UKSF being suggested before, it has come up in the past for American forces and as such this thread is locked as resuggestion.
Thanks

The key to modernising any weapon is covering them in glue and tossing them in a barrel of M1913 rails until they look "Modern" enough.
Many thanks to [R-DEV]Adriaan for the sig!