We have just had a new gas metre fitted by British Gas or one of their contractors and since they have installed this we now have hot radiators even though the central heating is switched off. We only have a back boiler van andyone give us any assistance as to why this has suddenly happened.
I sounds to me like a set of electrical contacts have stuck on. Not being familiar with your system I am only guessing maybe at the thermostat or a control relay near the boiler. Most likely the failure would not be due to the gas meter being replaced. May be someone more familiar with heating systems in the UK area can have more insight. It most likely will take some trouble solving to locate the cause after which it may be more apparent if it was related to the meter replacement. Ether way it obviously needs to be addressed. Keep us updated on the progress.