score:5
If nothing happened then and there, you're all good. However, you did indeed break the law, and had you been asked for it (you usually aren't), you would've faced a £100 fine.
In fact it's even required if not leaving the transit area. While check-in/gate staff mostly haven't been checking it, for my very most recent flight (from CPH), CFS staff weren't letting anyone without a completed form on the flight.