I've seen some UK flex that feels like doorbell wire but it's most definitely pre-metric (early 60s). I've got it on an old Robuk tape recorder but I don't remember what size fuse sits in the plug. I definitely didn't replace the plug with a Schuko because I didn't feel confident plugging this in without additional protection!
I've seen some 0.5 mm2 IEC C7 cables and they were very easily damaged so I can guess why the size isn't too popular. It's probably also borderline regarding short-circuit protection in countries with 20 A socket circuits like Belgium and France.
