This is all interesting.
My car is booked in next month for the last of the services on the current plan. I've been using Service Plans for the entirety of the car's 11 years (so coming up for an A service). I've convinced myself that the Service Plans (which I've taken for the longest duration possible each time) have been worth it for three reasons: protection against inflation, convenience and breakdown cover.
I'll soon need to decide whether or not to carry on with the Service Plans, or indeed whether or not it's time to use an indie since the car will be 12 years old by the next service. That next service (2024) is a B service, but not a massive one like last year's that included ATF and brake fluid replacement. However, it will have to include new spark plugs (6 of them). It seems that my local dealership doesn't do any discounting for older cars like Sandown mentioned above. I'll chat to them about what they can do when I take my car in next month. They've always done the MoTs for £35 so they may be able to offer something.
What I won't get by going to an indie is breakdown cover included, so that's an expense to be considered. They probably won't be able to offer a courtesy car either. (My local dealership is actually only a 20 minute walk away, but they always supply a courtesy car, which I see from the latest online stuff isn't a standard inclusion in Service Plans.) Unless my local Mercedes can offer something on the lines of Sandown's excellent discounts, it's unlikely that they'll be able to be as financially beneficial as the nearest indie (that's several miles away.)
Or I'll just take the lazy route and carry on with a new 3-year Service Plan at £44/month (compared to £36/month for the last 4 years).