When you look at it in a fairly black and white sense, if someone has been able to pay their (for example) £800 monthly rent for X amount of time, without missing a payment or relying on housing benefit, then it stands to reason that they can do the same for a £500 mortgage.
The issue is saving for a deposit while paying ridiculously high rents. I don't know whether that means a return to 100% mortgages for first time buyers, in the absence of rents falling or what, but I think if the deposit/rent problem could be eased, home ownership would become more achievable.
Also (sorry) but make life harder for the type of BTL landlord who buys up all the flats and two bedroom houses to rent out at extortionate rates, without doing any repairs, refusing to rent to tenants with children etc.
Also make five year tenancies an option