Euro 6.2 diesel cars are allowed into the charging zones so don’t worry about being able to travel anywhere you want, including Bristol(unless they ban all but buses but then it wouldn’t matter what you got).
Regarding mpg, people say that they got 40′-50’s out of their petrol by driving miss daisy, while this may be true if you drove a diesel in the same fashion you would be getting 60′-70’s mpg so in general even accounting for the price difference in fuels a diesel will be a lot cheaper over the 3 years.
Now, the BMW X1, a car that’s just had a facelift and it’s mostly cosmetic, no real changes in engines or dimensions, what you do get is the inability to adjust the height of the seatbelts?, Really? why did they think that would be a good idea? Are all BMW drivers the same size? The other great idea is to fit them with lots missing in the tech department and you have to use your phone to do the job of mp3 player and satnav etc via android auto/apple car play but now they are charging you £85 a year for the privilege plus a fitting fee of £235 at the time of ordering, I guess they think all their customers have more money than sense.