Academics in all fields should do the same.  Here is the very nice letter by philosophers Mary Leng (York) and Robert Stern (Sheffield), which they kindly gave permission to share:

Sir Alan Langlands
Vice Chancellor
University of Leeds
9/3/2018

Dear Sir Alan

    Re: Resignation of External Examiner Appointments, School of PRHS

Despite the rhetoric of ‘competitor institutions’, ‘USPs’, and ‘market share’ that
has encroached on UK Higher Education in recent years, the truth is that our
Universities remain built on a deeper foundation of cooperation, rooted in a
collective commitment to the advancement of learning and research. Nowhere is
this more evident than in the operation of examination boards, where, each
summer, academics from around the country, having emerged – often exhausted
– from the piles of marking generated by their own modules at their own
institutions, agree to delay embarking on their personal research agendas so as
to act as external examiners on the degree programmes of their so-called
competitor institutions. Yes, this work is paid, but as anyone who has spent a
sunny June day locked in a seminar room surrounded with endless piles of exam
scripts will attest, the continuation of this aspect of the quality assurance
framework depends on a significant amount of good will.

In recent years we have been happy to extend this good will to the University of
Leeds, as external examiners for MA and undergraduate programmes in the
School of Philosophy, Religion, and History of Science. But our good will is
primarily towards our White Rose colleagues in that School. Recent actions by
the University of Leeds administration, in particular the proposal to dock 25% of
pay for every day that an academic refuses to reschedule teaching that was
cancelled owing to the ongoing UCU industrial action, shows the contempt that
those in charge hold towards these colleagues, who in striking are taking the
only means available to them to challenge proposals that will leave them
significantly worse off in retirement.

Our good will towards our striking colleagues at Leeds means that, in light of the
actions of the University’s administration, we have taken the decision to
withdraw our good will towards the University. We therefore offer our
resignation from our posts as external examiners at the University of Leeds.

Yours sincerely,

Dr Mary Leng                        Professor Robert Stern
Department of Philosophy     Department of Philosophy
University of York                 University of Sheffield