If you only have £2.5k and need it as cheap as possible to run I’d focus less on mpg and more on reliability and/or never-go-wrong.
Agree 100% – Saving £10 on fuel per week would be wiped out in an instant with a decent repair bill if you are not handy with a spanner.
All about balance at this price-point, get decent economy and go for an acceptable but fairly undesirable car so if insurance / image doesn’t matter keep away from super-mini sized cars that first time drivers buy.
Put it this way, dad spent £1000 on an old Nissan petrol 2 years ago, he is still driving it, total repair bills = £60 on exhaust, other than tyres and oil change that’s it…. it wont do 50mpg but will do late 30’s / 40s so good enough.