Hotel or resort with indoor waterslides

Duinrell - weather on a par with UK really so shouldn't be too bad sun wise. Indoor water park, outdoor theme park. Nice town and beach in cycling distance, easy to get to Amsterdam for day trip if need be

Duinrell is fab.

I’d think centerparcs would be obvious, is there something about it that doesn’t fit what you need though?

Aquaworld in the north of Budapest. It’s so easy to get to - there’s a shuttle bus from the city centre or catch a taxi direct from Budapest airport. www.aquaworldresort.hu/en

Lalandia in Denmark is just 3km from Billund airport. Even though the waterpark is the main focus of the resort, there are lots of other activities to do too. www.lalandia.dk/en/billund

Sandcastle waterpark in Blackpool is massive. It doesn’t have its own hotel but there’s plenty of options for accommodation nearby. It’s right in central Blackpool, directly next to the South Pier and Pleasure Beach.

Well the obvious place is Center Parcs. Plenty in the UK or you can go to France, Netherlands or Germany. Lots to do on site, in fact my DC would happily spend every minute in the water park.
We've done the Dutch ones and prefer them to Duinrell but that might suit younger children. If you go to De Kempervennan it's near enough for a day at Efteling.

This one is fab and in/ out if you’d consider a uk holiday.

www.alpamare.co.uk/

Yorkshire has a lot going for it!

Alpamare is outdoors and not a resort just a day trip, the OP is looking for a hotel or resort with indoor waterpark.

Well the 3 different slides are definitely inside as is the wave pool. And isn’t it good to suggest a range of options?

Waterworld Stoke on Trent, new slides being built

Another vote for Duinrell. The pool is amazing, Beach is 4K away and pretty Dutch towns close.