Ye I think its spot on too. Good milage, looks really tidy. It also comes with a 1 year 5 star warranty which is worth at least £800 on that car. Trade and private are different markets.
As for BMW putting them through cheaper than this, ye maybe with 70+k on them, I haven't seen many for less than 15k AUC BMW and the ones that were had no spec and had high milage. But all BMW AUC cars get a full 1 year BMW warranty which is worth the extra IMO.
If you look nationally on BMW AUC there is only 4 and only one which is £15,250 other than that they are all £17,850 or more.
Z4MCs are rare and good ones are worth the money.