Hello Martin,
This is not meant to be critical or judgemental, but I believe you entrust your car’s care to the main dealer, because they are the best. By doing so, it sends 2 hours in their workshop, a little time in a valet bay, while you get your free cup of coffee - great - while your wallet gets twitchy about what is going to be extracted from it! Great, if that’s your thing and it gives you peace of mind, but bear in mind, they have work targets to generate income for that dealership.
Back to your rear brakes needing doing now:-
It would not surprise me at all to find that they don’t actually need changing now ( in caps), and if so no need to worry.
Your best friend for any ‘routine maintenance’ is a trusted local garage where you can talk with the chap who will actually be working on your car, rather than a front-of-house- service receptionist. My recommendation would be to try to find that garage, and ask them to let you know what brake pad thickness remains - as a starting point.
Kind regards,
Gareth.