Do you mean M Performance exhaust?
Is it the auto or manual?
It sounds like it has the most desirable options (pro nav and HK), but auto wipers, rear sensors, is all standard kit AFAIK.
FYI, when I traded in my 3 year-old M135i (facelift), albeit with 70k miles on but more options than the above car I got £12.5k for it. I guess private sale I might have got £14k.
I'd suggest the above car around £18-20k would be a fair price. They're not exactly rare, and discounts when brand new were massive.
Seeing as you're in Notts Sytner have these 2 in (both very similar miles):-
Used 2017 White BMW M140i for sale | PistonHeads UK for £20k manual
Used 2017 Black BMW M140i for sale | PistonHeads UK for £22k auto
he has a 17k p/ex offer so I think my offer will be same in cash