Realistically, on B roads, I'd suggest you'd be looking at around 35mpg. My old 2006 335i with the old N54 engine got 27mpg average, without driving it carefully, on A/B roads. I'd hope that a 2017 335i would be capable of more!
When you work it out, the difference between 35mpg and 27mpg when you're doing 9k miles a year isn't that much at all. Just doing some quick sums:
130p diesel 125p petrol
9000/35 = 257 gallons of fuel over the year
257 x 4.54 = 1167 litres diesel.
1167 x 1.30p = £1517
Same sums as above but for petrol: £1891
So circa £370 a year or £30 a month difference in fuel. It's £30 well spent IMO. The petrol six cylinders are lovely and are better suited for fun, B road driving. Plus they sound much better than the diesel.
Don't forget, this is the kind of car which will cost you £250 for a minor service and £450 for a major service and £650 for a set of tyres.