On a straight purchase 15k mileage wouldn’t be enough to offset the extra purchase price of a diesel. However, as most are aware one of the quirks of resale values (currently at any rate) tend to make diesel APs very favourable against petrol. So at least you don’t have that consideration.
City centre bans? I’m of the opinion that as most of these will/do rely on ANPR to allocate charges then (IMHO) the likelihood is road fund duty exempt registered cars only will be exempt, similar to many councils disabled bays now.
All in all, I think going forward if the vehicle you like at the price you like, is diesel then it’s still safe enough. I’m currently waiting for a diesel Sharan to be built.