Kindly see the embolded sentence. Why can't such a defendant rely on Necessity, but can rely on Duress of Circumstances?
2.5 Duress of Circumstances
In some cases where the defendant does the lesser of two evils he or she will be able to rely on the defence of duress of circumstances. For example, in Pipe v DPP86 a man was driving his ill son to hospital and broke the speed limit. Owen J held that the jury should have been allowed to consider whether he could rely on duress of circumstances (or necessity as he called it). As will be discussed in Section 5 of this chapter, duress provides a defence where the defendant reasonably believes that the circumstances are such that unless he or she commits a crime he or she or another will suffer death or serious injury and that a reasonable person in the same situation would have committed the crime. This defence clearly overlaps to some extent with necessity. Being faced with the threat of death which the defendant avoids by driving through a red light would fall under the defence of duress of circumstances; it would also be said to be the lesser of two evils. However, duress of circumstances is in some sense narrower and in some sense wider than necessity. It is narrower in that duress of circumstances is not available in order to avoid a threat less than death or grievous bodily harm [abbreviated GBH]. However, where it is available necessity may provide a defence even where the harm threatened is less than serious harm. Duress of circumstances is wider than a necessity defence would be, in that it covers the situation where the defendant reasonably believed there to be a threat of death or serious injury, even if there was in fact no such threat. A necessity defence would not cover such a situation.87 Further, if the defendant was threatened with grievous bodily harm unless he or she caused grievous bodily harm, although the two kinds of harm may be the same and so he or she may not be able to rely on necessity, he or she may be able to rely on duress of circumstances and argue that a reasonable person would have given in to the threat.
86  EWHC 1821 (Admin).
Herring, Criminal Law: Text, Cases, and Materials (2020 9 ed). p 670.