It’s not that buyers could afford a bit more, buyers could afford essentially to pay double in London.
This isn’t because of the deposit but because HTB essentially allows you to buy a property for double the price in London.
Most people couldn’t afford a 500K property not because they can’t save up £50K but because they won’t have a salary north of £100K for the bank to loan them the rest as lenders cap the loan amount at 4-4.5 times your annual salary.
So now people that earn £50K can buy a property that only people that earn £100-120K could regardless of their deposits.