"I teach physics, and one common GCSE question is "What is the mains voltage?" The "correct" answer is 230V, if you write 240V you'll lose the mark."
Tell that to Northern Powergen. When they retired our original 1954 single phase 25kVa pole mounted 400v to 250v step-down transformer back in 2014 they replaced it with a 100kVa three phase (only 2 phases in use) step down transformer and used the normal tap (250v). So for me up here in Northumberland, the correct answer is 250v (presently reading 242v as it's 6pm and all the neighbours are making tea.