Reasonable mileage. You lose some efficiency through the torque converter and extra drivetrain loss of an automatic, the extra weight of the folding top and extra frame stiffening hurt mileage a little, as do heavier larger diameter wheels as Brendan all mentioned.
Keeping your car in good working order isn't too expensive and is worth a few mpg more. Probably making up for the maintenance with gas prices in the UK.
The OEM paper air filters (the best) last between 15,000 and 30,000 miles depending on the dust and other particulate matter in the air. Fuel filters usually are good for 60,000 miles. Depending on what type of spark plugs you have, they should last between 30,000 and 60,000 miles, probably a bit longer if you have the more expensive 4 prong types. Oil is best changed every 7500 miles, oil filter is changed along with it. Try to use an oil with BMW approval as this is the best for protection and mileage. And lastly, make sure your thermostat is in working order. I think average lifespan of these is like 60,000 miles. When they slowly fail, you don't always get any messages, but they fail in the partway open position prolonging warm up times, which causes your engine to use more gas.