Clearly not permitted to use a green/yellow wire for anything other than earth, as a cable technically one could over sleeve one core, but since 1966 it has been required to run an earth wire even if not required in an installation, with the exception of a lamp suspended from a ceiling rose, so only when multi-cables are used can you over sleeve green/yellow wires.
So this needs correcting, and clearly both ends of the cable, and that leaves you with only two cores, so looking at moving where the main control is located. I had the same, which is why I fitted Nest Gen 3 as although the base has 9 connections in my case, the thermostat/controller only has 2, although not pleased with Nest as it does not link to TRV heads.
Be it wireless or wired, it needs a change. Many battery powered wall thermostats can work with two wires, but for wireless types, I think Nest e is about the only one where the base is battery powered.
Cheap wired is OK, but don't use cheap wireless, as it needs some fail safe, so if batteries go flat or the wireless connection is lost, the boiler will switch off.