Not sure how he has missed the Lake District. We have been numerous times and can recommend the C&CC site at Keswick and the CC site Conniston both down by the water.
Northumberland is a sparsely populated area but Alnwick and Bamburgh Castles are not to be missed although very different.
North Wales also worthwhile although we have not stayed there since the CC site at Carnafon was lost.
Not been to Sussex and Norfolk only once. Stayed at Sandringham and enjoyed that.
Motty needs to put these on his to do list and let us know how he gets on.