The MP5 assembled in the UK has serial number prefix that began with "EN". However it is unlikely that they were EN guns due to the early timeframe the SAS used the MP5.
No guarantees but I am doubtful they we anything other than German-built guns. In fact as I remember the GSG9 "loaned" the Brits the first MP5's.
The earliest co-operation between the SAS and GSG9 goes back to 1977 when Lufthansa Flight 181 (Op Firemagic) was high jacked and Chancellor Schmidt asked the then British Prime Minister of the time James Callaghan for assistance. After a meeting in London between the GSG9 and the SAS two SAS members were sent to assist them, they brought the then new G60 flash bang grenade. In St Augustin the GSG9 showed their Toys to the SAS. The next sighting of the SAS was the 5th of May 1980 (Op Nimrod) with their new Toys. As G3Kurz stated, but I would say they were Trails firearms rather than loaned, also to support this is the fact that there was more than 30 Troopers they had a mixed bag of MP5A3, SD and K’s. Also it would be highly unlikely for the Royal Small Arms factory to have the licenses, machines and tooling set up in less than three years.
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