It's also quite likely that if it were fixed wiring connected to a wall stat that it would be T/E (rather than flex), in which case it would definitely be non-compliant to use its bare CPC as a 'live' conductor, even if 'only' a neutral - and that regardless of whether there were some other CPC getting to the stat.Well that would indeed be non compliant unless you had an earth at the thermostat by some other route. You still need an earth even if it's not supplied by that flex. Unless it's a thermostat that's not part of the fixed wiring, but then changing the wire probably wouldn't be an issue.
Kind Regards, John