Normal recommendations would say it is too early to begin cutting in northern England, but my small ornamental lawn has started to grow already. Is it reasonable to take this as a sign I should begin mowing on a very long height, or let it grow?
Yes, everything that Peter4075 said - I've often cut grass in mid January here in the south of England if it needed it and the grass wasn't waterlogged or frosted. We have a variable climate where it's possible that Christmas Day can be warmer than a day in June, so grass cutting isn't really an 'only in spring and summer job' here. Keep the blades set high though,don't cut it as short as you would in say May or June. One old piece of advice I was given years ago from an ancient, experienced professional gardener was not to cut the grass in winter or very early spring if there was a sharp northerly wind blowing because it would make the cut tips brown. I found he was right!