All exam centres must have an examination contingency plan
This is explained on the website of the Joint Council for Qualifications (JCQ), a membership organisation comprising the 8 largest providers of qualifications in the UK.
The plan doesn't have to be trust-wide, but you should make sure all of your 'exam centres' (i.e. schools that offer exams with 1 of the providers listed in the JCQ regulations) have one in place.
Ofqual guidance
Ofqual explains that your school/trust leaders should:
... establish, maintain and comply with an up-to-date detailed written contingency plan, to mitigate any incident they have identified may occur.
Additionally:
It also sets out instructions for schools to follow before exams take place, in the event of disruption on the day, and what to do after an exam