The Green beret without much military knowledge I would immediately associate with British royal marines.
This beret is however khaki not green so probably from another regiment of the army.
A quick search online for khaki berets brings all these just for the UK alone, Foot Guards, Honourable Artillery Company, Princess of Wales's Royal Regiment, Royal Anglian Regiment, Royal Gibraltar Regiment, Duke of Lancaster's Regiment, Royal Welsh, Yorkshire Regiment, Mercian Regiment, Special OP Battery Royal Artillery.
Bt the way this a loaned air-soft gun, it's not real, although a pretty substantial weapon I have no idea how close it mimics the real thing.
Berets are cheap and easy to make and have been the default headgear for many military groups since the later part of the 20th Century
