I would say you have one of the lowest mileage Z4MR's around in the best colour combo, although that is subjective but Imoa Red with Black Interior demands top money.
Having new brakes and discs is a nice bonus, but provided you have a full service history it doesn't matter if the next one is an Inspection II or an oil service. People buying a Z4M must expect higher running costs than usual BMW's, and the cost of entry into the exclusive Z4M club. Still far cheaper running costs than Porsche or Aston Martin.
I would say your Z4MR is potentially worth £20k. What other roadster can you buy for that money that has as legendary an engine combined with exclusivity.
Be aware you have a rare and very desirable car, don't let it go too cheap. You just need to wait for the right buyer.
Too many people on this forum talk down the values of these cars, usually because mentally they are thinking of their own budget limits.
Put the price up higher and you can work down from that etc. If people bid you low just tell them to go and buy a 3.0si or a common Boxster!
Values will rise on these cars as you simply can't find many of them, and there are even less with sub 40k miles and FSH.
Good luck!