An update from my earlier thread:
https://www.theanswerbank.co.uk/Travel/Question1676141.html
This may (or may not) come as a surprise, but they made it to Spain this morning with no problems. The usual checks were made whilst leaving the UK and entering Spain and the passport was never questioned .... so it was either not noticed .... or no one cared.
It might be a different matter when returning from Spain back to the UK, so I'll post yet another update after they've returned :)