Hi OP, not read the full thread, so apologies if this has been suggested. First of all, I bloody love Mablethorpe! My parents have a caravan there, have since we were little, so had MANY holidays there over the years.
Have you looked at Butlins in Ingoldmells? I went there for the first time last October, and have now been 3 times. The first was me, my 16 and 12 year old, the second was full family (me, husband and 19 year old at uni as well as the younger 2 who are now 17 & 13). Then my 13 year old and I went again by ourselves in October. If you can get the dates right, it’s not massively expensive - and you can get a room or apartment rather than a caravan. Be warned, it’s a bit run down in the rooms (but might make your DD appreciate the caravan a bit more) but they have good facilities for the younger kids, loads of shows and entertainment for all ages, plenty of places to eat and drink, plus your a 5 min bus ride/20 min walk from Fantasy Island.
I think I’ve been fortunate that none of the boys have ever said they don’t want to come on a family holiday - my eldest is 20 now and he’s been talking about where we might go next summer, so he’s definitely still up for coming! I don’t know what the trick is to getting them to ge enthusiastic though, I think I got lucky. We only ever tend to do UK caravan style holidays, or if we can get a very cheap week we might go to Center Parcs.