With the whole of the country to choose from, it's hard for me to know where to start!
However Summerfields Holiday Park (which is a Parkdean one) in Scratby, near Great Yarmouth comes to mind. I've stayed their myself, at the invitation of friends, and I was quite impressed with it. (It was at Christmas though, and quite a long time ago, but it will at least offer a starting point on prices and facilities).
Details of the holiday park can be found here:
https://www.parkdeanresorts.co.uk/location/east-anglia-lincolnshire/summerfields
(Click the tabs for 'Accommodation', 'Facilities', etc)
The beach is just down the road, and pictured here:
https://www.thebeachguide.co.uk/south-east-england/norfolk/scratby-beach.htm
For a 3-night break in the school summer holidays, starting on Friday 30 July, Summerfields is offering a 'Caister Maisonette' for £469. That works out at about £26 pppn. It sleeps 6, with two double rooms and one twin room.
If you can go later though (after the school holiday rush has ended), starting on Friday 3 September, the same accommodation is offered for just £179. That's only £10 pppn; I doubt that you'll find accommodation much cheaper than that very easily!