Of course there's a defence!
(3) It shall be lawful for the Rules of the Air to be departed from to the extent necessary:
(a) for avoiding immediate danger;
...
(4) If any departure from the Rules of the Air is made for the purpose of avoiding immediate danger, the commander of the aircraft shall cause written particulars of the departure, and of the circumstances giving rise to it, to be given within 10 days thereafter to the competent authority of the country in whose territory the departure was made or if the departure was made over the high seas, to the CAA.
Get your pen out.