Be aware that a service plan will only cover those items which be due at a particular point in time, it will not cover items which may or may not have been done in the past.
I’m sure the main dealer can quote for each item may have been missed and charge you for those separately, although the risk is that they will be done again prematurely under the service plan in future services.
For example spark plugs will probably be four yearly. If you do them now, and they’re due next year then then they’ll be done again next year even though they’ll not actually be due for another three years.
If you’re thinking a service plan would be right for you, then find out what would be due in the next couple of years and take a view on what to do now or not.