I think Paultons Park complements Merlin very well with it's high quality approach to theming, maintenance and experience.
It's a more premium family park which in a few years will continue to grow and evolve especially with more thrilling rides coming with the new Viking area.
Comparing Paultons to the UK Merlin parks is like comparing Phantasialand to either Heide Park/Gardaland or the Walibis.
Alton Towers and Chessington both have similar or worse planning constraints in comparison to Paultons Park so I think they'd compete with those parks more easily than they would with Thorpe Park.
I've got the view that Paultons will compete with Merlin and Alton Towers even if that's not the intention. It's in a more accessible location than Alton Towers and is close to Bournemouth, Southampton, Portsmouth and Isle of Wight which are popular tourist destinations.
I am very excited about the park's future.