That would have been mine!
I think the main reason you thought it was overpriced (which it's not) is that you placed no value on the winter wheels (Style 65 so worth a fair whack by themselves) and that it had the NBT retrofit from the F10.
As for the Imola Red M5 in the OP, there aren't many Imolas to pick from (79 in fact). Spec is fairly standard and if the service history is good then I think £18k private is achievable but then, who knows? I'd be tempted to stick it up for £20k and see what happens. You can always reduce in price, you can't go up!
Pricing for BMWs of this age is always tricky.