Best thing to do, is decided on the area you want, and then go on the campsite search. Not sure but in the refine search bit you may be able to add pool wanted.
The read the reviews.
This is what I do. play area/dog friendly are two things that I look for.
Plassey at Wrexham has one. Not quite the north west but not far from it. If you are looking for an all singing/dancing site there are a few resort style places in Cumbria and Lancashire too, just have a look on the hoeseasons sites for ideas.
Another one that is a bit pricey but is a lovely site is Waren Mill near Bamburgh in north Northumberland. Near both country and beach it really is a gorgeous area.
Our sister park Crows Nest is next door to Flower of May and also has a swimming pool with a water slide. We have new touring super pitches and a new toilet block for the tent field this year too!!