What mileage should I choose?

That, unsurprisingly, depends on how far you think you'll be driving. Knowing how many miles you normally cover in a year is a good starting point. If you don't keep count, check the mileages on your old MOT papers. Or, make a list of the most regular trips you take in a month. Multiply that out over the year and add a little extra for ad hoc or special trips (like the annual drive to see Nana in Newcastle).