After lots of searching it looks like Park De Haan in Belgium which is currently under renovation will be re-opening May 2020 (hopefully will be all new and shiny).
A comfort cottage sleeping 8 (I've booked for 9 with an infant) on 24th August - 31st August and so covering bank holiday weekend comes out at 1560eur. Other weeks in August are still available. The premium cottages are 120eur extra, I didn't think it was worth it personally.
The Dover to Dunkirk ferry is approx. £120 per car of 5 and the park itself is a 45 min drive from Dunkirk.
Book through centerparcs.fr/
and it comes out 90eur cheaper than the GB website. You just have to use Chrome translate to turn it to english if you can't speak French. If you book early you also get 30eur vouchers to spend on activities.
The equivalent week in a UK centre parks is approximately £3500!!!!!!!