Following the Gulf War of 1991, the UK formed a shadow PSYOPS unit called 15 (UK) PSYOPS Group (Shadow). A double digit prefix was selected to avoid any confusion with the U.S counterparts of the 2nd, 4th, 7th, and 8th MIS/Psych Ops Groups
https://www.army.mod.uk/who-we-are/formations-divisions-brigades/field-army-troops/77-brigade/