I don't think the penny pinching NHS would offer it free if they didn't find it cost effective.
I've had it every year for the last 5, with no ill effects. My 90-year-old mother has it too, on the grounds of age,not diabetes. She does always have a bit of reaction, but nothing serious.
I once had flu, many years ago and was VERY ill. Remember the post world war 1 Spanish Flu epidemic, is supposed to have killed more people than the war! and they haven't got an effective anti-viral if you do get it.