That would make it an unpressurised cylinder and, therefore, would
almost certainly make it legal, but it would require a header tank
and not provide mains hot water.

Er. not so. There should be a a vent pipe coupled to a pressure valve.

Well, I would call that a pressure relief pipe, not a vent, but it's only a
matter of nomenclature.

After all, they are called "unvented" cylinders, which sort of implies there
is no vent.

Christian.