I have never brought a service plan because I shop around and try to get a cheaper service by asking, "Do you offer discounts?" With a plan, you are tied in and no room for discount. Having said that, a service plan allows you to budget and plan on the financial side.
As for the plugs, mine was done at 65k miles by my local independent. Yes, they were around £50 but was fitted as part of the service. The gearbox oil was done at 75k miles . As for technical questions, you better ask for the technical department as the staff at customer service are non technical. Why don't you try calling another Honda dealer?