affordable is a slippery concept that, for individuals, can change in a blink of an eye if interest rates change.
You're right Cunco it's in the main regional. Anyone who lives out in the less affluent fringes of the UK and has been to London recently will have noticed that it is continuing to burgeon with building sites, usually constructing places for people to work in, all over the place. So where are all these extra workers going to live? The only alternative to building in green belt (which in the case of London includes Epping Forest to the east, the leafy North Downs to the south, the iconic Thames valley to the West - that view from Richmond Hill! - and the Chilterns to the NW). So a lot of really special landscapes.
The sane answer would be 1/ a policy to get some of those public and private jobs out of London. The whole of Network Rail offices were for instance moved from Euston to a new building in Milton Keynes, and 2/improve transport from places a little further away. And making it cheaper. However train capacity from places like Brighton, Luton and Milton Keynes is not good enough or cheap enough to make commuting a realistic solution for those in lower paid jobs.
I hate to think what kind of overcrowded accommodation there is in London - I can only presume that people are living with a dozen people in a house suitable for 3 or 4.