Get a quote from the MG website. You enter your location and it offers you local dealers. Select one, specify number of miles per year, how long the plan is to last and extras, eg MOT. The plan is more expensive than the sum of the services and the 4 year plan is very expensive. Must be assuming severe inflation after 3 years.
Correction: Just read the contract. Not inflation but a management fee of £1.20 over 34 months - I chose a 3-year plan at £470.19. I looked at a 4-year plan but rejected it because it seemed very expensive at £743.16 - the 4th year service was over £200 so perhaps inflation after all.