(BBC serial, 1985) Starring Dennis Lawson and Sheila Gish and based on the Kingsley Amis novel of the same name, “used a repainted London RT in South Wales Transport livery for bus scenes” (thanks CCR)
Dennis Lawson boards the bus at night in traditional Welsh costume. The vehicle in question is RT3143 (KYY872), and it was repainted in London Transport livery afterwards: