Posted: Fri Feb 26, 2021 4:32pm
Hi Tim,
You're talking about Schengen rules. As a Third Country National resident in the UK, you can't spend more than 90 days in a rolling 180 days in the Schengen zone. Similarly, your parents, as UK citizens, despite their legal residence in Spain, have to comply with the 90/180 rule when visiting other Schengen countries. However, the UK is not (and never has been) in Schengen and, as UK citizens, your parents can stay as long as they like in the UK, but in their case, for visits to the UK, it's Spanish residency law that is relevant, not Schengen.
Kind regards,
Kim