Once informed they make you have a short, independent medical examination, which includes a urine screen for illicit drugs. If there is only prescribed oral methadone in the urine they will probably issue you with a licence for one year, but if you are on injectable methadone they will withdraw your licence because it is more sedating. You will be called back for another medical when it needs renewing (or when you reapply) and every year until three years after your script has stopped.
Regular users of cannabis will test positive for up to a month from last use. If the test is cannabis positive they withdraw the licence for six months. If other illicit drugs are found they remove the licence for one year. There is another medical on reapplication for the licence, and every year for three years, once it has been returned.
It is an offence to be in charge of a vehicle when 'unfit to drive through drink or drugs' - this includes prescribed ones. if you do carry on driving on a script, take care and don't drive if you feel sedated or if you have had any alcohol.
If you are involved in an accident and your insurance company finds out you are on methadone they could claim that it invalidates your insurance. I/