I never quite understand when people say they can't afford to holiday in the UK
Afford is probably the wrong word, more that people don't think it's worth the money.
If you go to the Med, Canaries etc in the summer you are virtually guaranteed good weather. So you can fill your days with free activities like outdoor hotel pool, swimming in the sea, sitting outside in the evenings with a barbecue etc etc and it feels like a nice break.
Plus as you know you'll be spending very little time inside your accommodation, you're less concerned about how nice, comfortable it is.
Accommodation, attractions are generally cheaper, as is eating out and drinks, and you rarely have to pay for parking.
We went to what is probably the Mallorcan version of Lands End (Formentor lighthouse). No entrance charges or parking costs and the cafe served good quality home made food extremely cheaply. I think we got two drinks, a pastie type thing and a cake for under 5 euros.
Compare that with UK Lands End, where you have to pay to get in, pay to park and will pay two or three times the price for refreshments. I could give endless examples like this.
Plus with the unpredictable British weather, you're likely to do more paid attractions to get out of the rain and less likely to be able to sit in the garden or terrace for most of the day and evening, so you're all crammed into the living room of your holiday cottage, caravan etc that you've paid thousands for and at that point, you start to think that you could have saved the money and stayed at home to do the same for free.
Plus there's the crowds, because like a PP says, there just isn't the capacity for everyone in the UK to go on holiday here during peak periods. SItting in traffic jams, queuing and being jostled by crowds does not make a pleasant relaxing holiday experience.