They could always close the park if they want to a big rebrand instead of replacing sections at a time. They would still probably be able to get this up and running before whatever they'd build in the UK, and that's assuming they get all the necessary approvals to build there.
I just can't see a park in the middle of the UK (without Potter, to add) being more popular than a park in a sunny resort town in one of the most visited countries in the world; it's really a no-brainer imo. There has been a lot of discussion about all the infrastructure upgrades that would need to happen in the UK, but like... they're already there in Spain. There's an airport literally 10 minutes away.
Maybe a smaller park similar to Singapore could work in the UK, but for a true destination park Spain seems like the better bet imo.