My daily commute is about 10000 miles per year, so I will assume something in the range of 15000 miles per year max.
The problem you'll have if you went the diesel-engined route is that you cannot guarantee (whatever the current mileage shown on the car is when buying it) how the car was driven all throughout its previous ownership(s) - all it takes is some time with it having been driven for repeated short trips from cold and that means the DPF and other related bits (including related to the oil level rising via unburnt diesel getting into the system via the sump) going wrong in short order and in a very expensive way.
It's essentially a lottery, hence why getting a petrol-engined car for mileages under 20k pa is the more senesible way to go. If you're lucky and YOU don't do mainly short trips and the DPF regenerates passively all the time (or you can tell somehow if it's actively regenning and don't turn the engine off when it is), then you'll save money if it stays reliable. If not, then it'll end up costing you a LOT more, even accounting for the better mpg.
Also bear in mind that if you live in and/or regularly drive into/through a larger town or city that has (or soon will do) an ULEZ (e.g. london, others soon to follow suit), which will likely restrict diesels (without very large daily charges/extra congestion charges or even band them outright) to at least EU6 or better (the gen-2 Mazda3 2.2TD is only EU5, the gen-3 1.5 & 2.2TDs are EU6), whereas petrol-engine vehicles are restricted to EU4 or better (and all petrol Mazda3 and 6s [at least from 2004] are EU4 or better). I'd check Honest John's website for details to see all about these Good & Bad things and the emissions details:
https://www.honestjohn.co.uk/carbycar/mazda/
