You could go down the Kings Road to BOY and buy something in the ‘Seditionaries’ range. A long-sleeved muslin shirt with two layers of fabric, D-rings and dog clips. Nice. As worn by the Sex Pistols who’d split up four years before but lived on at BOY in their God Save The Queen T-shirts.
I never set foot in their Carnaby Street store – I’m assuming this opened after the Kings Road. Unless you recall better.