This is what BALPA told their UK members regarding strikes in other countries:
CAN WE JOIN THE OTHERS STRIKE?
Not lawfully, no. Any UK Ryanair pilot that takes part in another country’s strike in any way would be in breach of their contract of employment and liable to dismissal.
WHAT ABOUT IF RYANAIR ASKS US TO DO THE WORK OF STRIKING PILOTS?
The natural reaction is to support colleagues of course. However, to avoid potential dismissal for breach of contract, UK Ryanair pilots should continue to work in accordance with their contracts. This means that Ryanair can ask UK pilots to cover for striking pilots and a refusal could result in dismissal. It is important that pilots are not inadvertently caught out as a result of a wish to try and support their colleagues in other countries.
IS IT DIFFERENT FOR CONTRACT PILOTS?
BALPAs understanding is that IALPA has not included contractor pilots in its ballot so they will presumably be working as normal. Here, in the UK, contractor pilots should also work as normal in accordance with their contract.