Are you sure your friend paying £320 isn't on a normal commercial contract? I rented with Peabody for a few years and paid £1100 ~ £1300 for a 1 bed flat, but it was full market rent. I didn't have any of the same rights as the other tenants in the block. I believe it was a way for them to earn hard cash.
£320 for a two bedroom sounds "affordable" for wide parts of SE1 (given private sector rents are probably £400+). Your friend paying £90 is likely on social rent, which is rarely offered outside of Council Housing. Social Rent requires you to register for Social Housing, while Affordable Rent properties can be rented directly via the Housing Association / Government Website and have relatively loose criteria (hence apply to a wide range of households).
Years ago council owned property used to rise with rate of inflation plus 1% , don't know if that applies now. Housing associations used to charge what they liked. Peabody thirty years ago would not house anyone earning over £12,000. Wonder what rates apply now?
Both of my sisters rented from the same housing association. One had a 3 bed house with garden, the other a 2 bed flat. Within a street of each other and the 2 bed flat paid almost £70 more in rent.
My rent has increased so much in the last few years, no improvements made, if anything has got worse. If you look Southwark's bidding website rents for HA properties (market value!!) are almost double that of council ones.
Pops at school we were given lessons on how to work out our budgets for every day living...we were told to allow One Seventh of our income for rent! My full rent is almost Seven thousand p.a. According to thos figure I would need £49,000 in my hand p.a.!
Wish my old teacher in economics was still around!