I will defend Cornwall, we have holidayed there many times in all seasons and weathers. Most recent this August -probably our best holiday yet! We camped near Land's end and found it very easy to get away from the hordes, heading to the quieter beaches some you can only walk to. Kids had wetsuits when they felt chilly. Paddle boarding in quiet harbours, kids tombstoning off piers at high tide.
Got out early before too busy, eg went over to St Michael's mount as soon as it opened = hardly anyone about. Crowds were arriving as we left!
Day trip to Scillies, yes the boat was busy but as soon as we arrived we took a small boat to a different island - hardly anyone on the beaches.
We like all the history in Cornwall, so we will use our map and explore ancient burial chambers, standing stones and old mine engine houses and museums. We booked a mine tour which was more like caving, lots of ropes and abseiling - kids loved it.
One day of bad weather = museums, ancient churches, cosy cafes, cinema and driving round to look at the crazy waves at an empty Sennen cove. It's really not hard. Get a map and walk to interesting places! If you only go to the tourist honeypots of course it'll be busy. Sorry rant over!