1) Significantly higher taxes on the rich, and a total clampdown on tax evasion and avoidance - essentially 'if you trade in the UK you pay tax in the UK', not any of this nonsense that Amazon and the like pull by pretending they're a company from Luxembourg.
2) Investment outside London.
3) A huge number of local authority homes need to be built that cannot be sold, under any circumstances. Not 'affordable housing' because the problem is simply too bad - the demand will always mean what is 'affordable' today will in 5 years have made whoever bought it 50 grand and now be unaffordable.
4) End austerity yesterday and absolutely throw money at public services, both on a national and local level.
Pretty much every problem in the UK at the moment has been caused by three things - abandoning everywhere outside the south east and London to rot, structuring the entire economy around property and absolutely chronic underinvestment in public services.
The problem isn't so much that there are too many people, it's that the people that are here are being taken care of so poorly by a ruling class that doesn't give a damn and are being overseen by an opposition that doesn't either.