UK Planning law works differently to America. They would have to announce before they get the ok to build.
We would have fully fledged plans submitted with a public consultation period where people (who probably dont live there - READ: rival Park operators, or are affected by the park's existant) objecting to it. I could go on at length about MSG Sphere and how AEG did a great job of lobbying in London to get the Sphere stopped.
If Universal are smart, they will chose a nice greenfield site surrounded by very little maybe a few farm houses that wont be affected by a major themepark resort, with infrastructure (trains and major roads) links to major cities and a town closeby that needs some regeneration and a county council that is very much looking for major investment and significant jobs, who would be happy to greenlight the project. This location would have to be far enough away from any of the bigger Merlin Parks so they have very little leverage in stopping the park being built. So basically, 70% of the entire country is in the running, the mistake would be to try and build it inside the London M25 corridor......or anywhere East of Thames Barrier