I read this with great interest! And figured out that every time I went to the UK, the weather was extremely good! The Autumn of '78 (apprentice in London): hot. With my wife to Scotland: dry and fair (late October). The Olympics in 2012: hot. And the UK Laser Championship in Devon, July 2018: scorching hot.

On my to-do list in the UK: sailing the Norfolk Broads with a traditional yacht.

You write that your kids are between 10 and 18 y/o. I know from experience that it's not the easiest age bracket for a cheerful holiday... I would certainly pick something that is a highlight to visit: London is one of course. Lots to see and do for older kids, but a camping near Bovisand or Down Tomas, southeast of Plymouth isn't very alluring for youngsters that age...

It's also worth checking if a crossing from (for us) Duinkerken to Dover isn't a better option when you write that a ferry crossing isn't high on your wife's list.