There are some really cool british weapons, and I think they’d make a neat addition to the game (and I would certainly buy them too).
SA80 - bullpup assault rifle, afaik there are no bullpups in the game currently and it would also add a more immersive option for clearing bunkers and buildings for example where a shorter barrel is an advantage (in real life at least) (5.56x45mm so uses the in game 5.56mm round)
L96 - bolt action sniper rifle (7.62x51mm NATO so would probably use the generic 7.62 round)
L86 - light support weapon, which is essentially an SA80 with a longer, reinforced barrel, but in game I would interpret it like another machine gun with a slightly more “assault rifle type look” (also uses the 5.56mm round)
L128 or Benneli M4 12G shotgun - semi automatic shotgun, which I would love to see in game
L74 or Remington 870 12G shotgun - used by the SAS and manufactured in America, which in real life is used with special rounds to create localised damage for destroying door hinges and locks, so could be designed to only use slugs (if it needs a special feature other than being a really cool weapon :P)
L129 sharpshooter rifle - rifle intended as the designated marksman rifle, so would likely function in a similar role to the S21 that’s currently in game (uses 7.62mm rounds)
L90a1 - essentially another MP5 but with a shorter barrel (9mm rounds)
L7 gpmg or L110a3 minimi - machine guns, the minimi is older and no longer in service so may also fit the time period better as I’m not certain when the L7 was first used
Any one of the many anti material weapons (rocket launchers in game) including the NLAW, Javelin, Carl Gustaf M4, or for my personal suggestion, the Starstreak - an unusual weapon that accelerates the projectile to mach 4 then releases 3 22mm sub munitions, which in real life are laser beam guided but in game would likely be unguided
Sorry for the big wall of text of weapons,I tried to omit as many as I could bring myself to but there are many many ones that I would love to see in game, I hope it gave some good ideas and will be considered.