Essentially, it's about the defendent having committed an offence based on the situation he is in that compelled him to choose the lesser of the two evils, should it be allowed?
The English courts have differing judgments and in most cases they have been hesitant to apply it. I find it most interesting in cases of assisted suicide and euthanasia where the prospect of such defence is likely to be used.

In the case of R V nickling, a man was paralysed all the way from his eyelids down and lead a harsh existence. He felt his article 8 Echr rights, (right to family life) were being violated. The court held that anyone who assisted him in the act would be criminally liable and such defence would be unsuccessful.

In the case of Airedale NHS vs brand, the court went on to say that the defence of necessity could be used if the doctor omitted to treat a patient if it was deemed to be in his best interest and any continued treatment would be of no benefit to him.

This defence was further allowed in the case of Re A, wherein a mother gave birth to a set of conjoined twins and the doctors stated that the weaker twin would inevitably die, and yet if they were not separated, the other twin would die as well, the doctors against the consent of the parent obtained a court order to continue the operation based on this very defence whereby the court states that 1. It would be justified if there was necessity to avoid an irreparable evil. 2. No more than what is necessary was inflicted to achieve the purpose. 3. the evil inflicted is not disproportionate to the evil prevented.

In the case of Re f, the court even allowed the sterlization of a mental patient who was found to be having sex with another patient at the institute, fearing she would get pregnant, the court stated that the sterlization was justified on such defence as she was not able to appreciate the nature of the conduct and give valid consent and it was in her best interest.