Yup, that's exactly what I decided to do - paid for the first A-service in full then bought the 3-year plan to cover the next B,A and B services. Sounds like there is a promotion on at the moment with that £31 p.m. offer, I signed up at £35 for my 220d Sport as soon as I'd had my 1st service back in June 2017. I vaguely recall I could only take up the 3-year plan provided it was within the first anniversary of the car's registration date. I don't know if that was an MB UK stipulation or just sales leverage applied by my particular Stealer.