You may have seen a letter to the Guardian from a group of professors explaining why they are resigning as external examiners and encouraging colleagues to join them in doing so, see

https://www.theguardian.com/education/2020/jan/22/we-refuse-to-act-as-external-examiners-until-universities-deal-with-staffing-issues

UCU have provided guidance for staff considering following suit as to how to do so within the terms of your external examiners contract: https://www.ucu.org.uk/externals

This is one of many areas where academic staff act essentially on the basis of goodwill to benefit the whole sector and such goodwill is being seriously eroded by employers’ responses to the current two disputes on #USS and #FourFights.