I have been on loads of UK holidays and have rarely encountered weather so bad that you can't go outside! On those rare occasions we've always found an indoor attraction, e.g. museum. caves, so not wasted
Fair enough, but I see the point that if the weather is awful, which it could well be, even in the 'summer' - we've had an awful lot of cold rainy weather in August in recent years, making the best of it outside gets old after a day or two and most indoor attractions in the UK are expensive, in addition to most leisure activities in the UK being more expensive than abroad, it can seem like a waste of money.
It can be cheaper to go abroad than somewhere like a cottage in Cornwall, eating out, petrol, public transport all cheaper, you rarely have to pay to park and you can reliably fill your days with beach, pool, walks etc that cost very little and you won't find yourself paying £££s to get out of the rain.
I wouldn't say I would never holiday in the UK and am this year as it happens, but it is an activity holiday that is weather dependent. If the weather is against us, it is literally £1k pp down the drain and I could do the same thing abroad, probably for less, with almost zero chance of weather stopping play.