I suspect some of it could be people who bought in a cheap area of London 10-15 years ago who have realised they're sitting on more like £500k of deposit (and obviously high income too). Some areas have more than doubled in price in that time.
Yep, flats around me were going for £250k ten years ago and not far off £500k now. Once you knock off the original deposit and ten years of repayments you're releasing £350k or so of equity and potentially looking at getting up to 60% LTV and the deals you can get with that.
I suspect some of it could be people who bought in a cheap area of London 10-15 years ago who have realised they're sitting on more like £500k of deposit (and obviously high income too). Some areas have more than doubled in price in that time.