Hi Steve,
The problem is now that virtually anything inside the M25 seems to be in the ULEZ zone and I do travel inside the M25 pretty regularly.
I can probably currently stretch to £15k for a car at the moment. For that price I can get a low (ish) mileage 2012 to 2015 TDI cd but it won't really buy a late 2015/2016 car as they seem to be about £5k more for an equivalent car. I plan to use the car for towing so a petrol engine version would not be as good.