We went to Centre Parcs off season / out of term time and it was say it was fine (it didn't cost more than an air B and B although obviously we spent quite a bit whilst there).
My DC would love to go back but now they are at school it is (I reluctantly checked) three times more expensive and absolutely not worth it just for the pool - which is the bit my DC loved. And we were not enamoured with the rest of it tbh.
Where else is a good pool with slides / holiday vibes?