"...short 10 round STEN magazines are an American thing..." Nope. CF had 10 round mags for our Sterlings. Armoured crew used 'em. I'd be very surprised if the STEN didn't have a 10 rounder for the same reason. Nothing to do with the U.S. or India. And Stateside a STEN requires the Class 3 permit regardless of where they are. Or if it's still FA or not. Daft State mag capacity laws are the least problem. Gunparts will sell you what they say is a "surplus" 20 round STEN mag.
Don't get your training on YouTube. Anybody with internet access can post any daft nonsense they want there.