Nick, most of the experts on this forum will advise not to prune any citrus under most circumstances . That being said, I visited a local Citrus grower who regularly prunes his trees so they remain compact for his cold frame. From what I saw, he still has plenty of fruit production from his trees. He only ads heat once the temperature hits freezing or slightly lower ( to save the fruit)
By the way, I am no expert
Where in Britain are you?
Greg