I import more than I export in December and January, but for every other month of the year it’s the reverse by some margin. My annual electricity bill is net negative, and the installation cost will be paid off in 7 years. The numbers would be even better in, say, Spain, but they are perfectly adequate here, despite suboptimal latitude.

Incidentally, the grid only knows about my exports. Self-consumption manifests as demand reduction, so UK solar production and electricity consumption is probably higher than aggregate figures suggest.

True, but if you get cheap power from solar for the rest of the year, and charge the battery from the grid off peak when there's no solar to charge, you should end up paying less than just the standard rate.