Adrian,
A nice idea but..........
That would be socialism. Well, it would actually be state sponsored banking and although I don't deride the idea per se, I'm not convinced that the public would back mass lending to people that, in many instances, would otherwise be regarded as subprime. That said, Karl Marx did, to the belittlement of capitalism, predict that capitalist markets (and capitalist governments therein) would always find a way to make the poor keep buying.
Start effectively offering £150,000 houses for £15,000 upfront and those houses don't stay at £150,000 for more than a minute. Well, they don't in a market economy anyway. It's just a mortgage with a 10% deposit which, instead of being funded by the banks, is funded by the taxpayer.
And anyway, try buying a property in many parts of the country for £150,000. Again, an increase in supply is the only viable solution if housing prices are not going to continue to exponentially outpace income.