IME, nothing would happen if the ‘friend’ didn’t want it to.
We used to let a house, and one tenant failed the credit check, but offered to pay a year’s rent up front. The agent talked us into it, saying it would be fine because we could give them notice towards the end of the tenancy. The fact they failed the credit check meant we couldn’t get the landlords’ legal insurance which we had had before. It had proved very handy, because the previous tenants had stopped paying the rent. The insurance covered the unpaid rent less half a month, and the legal process of evicting them.
So when thy defaulted on the rent after paying just one month after the end of the year they had paid up front for (the agent conveniently forgot to issue the paperwork about renewal or end of tenancy), we had to engage our own solicitor to go through the process and evict them. We simultaneously took them to the small claims court for the unpaid rent, which by the time the legal process was exhausted and they actually left, was around £6K. They also trashed the place leaving us with a £4K bill to put it right and redecorate, plus a £1,500 bill for the solicitor.
We engaged the High Court Enforcement Officer to enforce the CCJ. But the ex-tenants were conveniently never at their parents’ address where they were allegedly living and had no possessions, including the car they were driving round in - that belonged to the parents. Strange, really, because they had had furniture and possessions when they lived in our house. So they had no assets which could be seized and apparently no earnings which could be attached to. So we never got a penny.
£11.5K we will never get back. It is because of people like this I would never again rent to someone who wasn’t completely belt and braces gold-plated able and willing to pay their rent. Fortunately, the next tenants were lovely, looked after the house, paid the rent on time for 8 years and then bought the house from us.
I also have left-leaning principles, and people need places to live. However, social housing is the role of government/charities, not of a landlord with one property, which was actually their own guarantee of a home in retirement, being as they were themselves living in job-related accommodation. If I wanted to accommodate people for free, I would do it. I didn’t - I offered a lovely well-maintained house for a fair rent, and the CFs took advantage of it.
So, OP - if you want the warm fuzzy feeling of providing accommodation for someone who can’t otherwise afford to live in London, crack on and let ‘friend’ into your home. But don’t surprised if you don’t get either the minimal rent you’re asking for, or any of the £4K back. And don’t say you weren’t warned.