Tom,
I had for many years a service plan for my Jags and swore by them as it spread the costs over the year so did not get a big bill once a year. However, when I changed main dealer I was persuaded to go for Fixed Price Servicing (by the sealer) as it worked out much cheaper for the same thing.
I now do the Fixed Price servicing, yes I have a bill in one hit but at less cost.
Worth chatting to your main dealer. Hope this helps.