I agree, its just the sum of 3 years worth or 30,000 miles worth of services at todays price fixed for 3 years.
For me, I know the price will rise with inflation but I would rather pay year on year and have the extra cash in my pocket today to pay for other expenses. If you have £1300 to spend that you won't miss then go for it. I think the small service is about £250 and the big one is about £590.