The Regiment is old, but perhaps best SAS related tactical first person shooter to day.
I used to play it years ago, and would do so again if there were any servers online.
It's a game about SAS (Official nickname The Regiment) and it's based on real events such as Iranian Embassy and London Metro.
It's like Swat 4 and embraces tactics above all. However, it's focused on fast movement and more brute-force approach to situations. It's fast paced, but surprisingly easy to adapt to.
It has the best tactical fps training set I seen in any SAS related game, and if nothing else the game could be used for your personal training and familiarization with SAS tactics.
The Regiment is game made in 2006, so it's graphics are a bit low-end, but still playable. If you want enhancements, you can use it with ENBSeries mod.
The Regiment does support mods, and there are few to use.
The Regiment Gameplay:
If nothing else, it's worth a try, if you haven't played it yet I suggest you do, even though MP is dead, SP mode is fun and replayable.
Speed, Precision, Experience, Endurance. General System Tweak Guide