M40A1 or the IW-80 from the socom series as well.
That and i kinda feel like every shooting game requires the AK-47. Maybe that's just me though.
The AK-47 would lend itself well to the story if you ask me, its a very simple gun to make. It has something like 7 moving parts, which would be a bonus when you decide you need to militarize your police force, and need to make them weapons.
I personally like the L85A more than the SA80 IW, they both came from the same development team to be used together, but the L85 just looks so cool with the HK grenade launcher. http://world.guns.ru/userfiles/images/assault/as22/sa80-l85a2_ubgl.jpg