Landlord is not a criminal
Does not have rats
Not a council house
Was not previously occupied by criminals
It doesn't have elderly neighbours
No kebab / chicken shops within x metres
Or do you really mean not a council house, in which case the question is "what letting agents are advertising current council houses?"
The Deposit Protection Scheme was introduced in 2007, and that made a huge difference. Landlords could no longer expect to steal deposits.
There's already schemes to license landlords renting a "House for Multiple Occupancy", whose definition varies by area. Typically, it's 3 or more people who aren't related to each other. This should be extended to all landlords.
I lived in an illegally converted flat for a year, though I didn't realise it. At the end of the year, an inspector from the council appeared, and said there shouldn't be a "37A", "37B", "37C", but just a "37". The house had been converted without planning permission _fifteen years earlier_.
3 million Londoners weren't born in the UK, and in my experience are less aware of the rights they do have. I don't know how to solve that one.
But 39% of Tory MPs, and 22% of Labour MPs, are landlords, so I don't expect the situation to change. [1]
[1] http://www.theguardian.com/housing-network/2016/jan/14/mp-la...
Agents are not incentivised towards keeping listings fresh. It's better for them if they can tell the person that gets in contact that it's "just gone" and hem-and-haw about how the market is very hot at the moment. They can then show you a selection of over-priced, not as attractive properties and try to extract a purchase from a slightly more anxious version of you.
I'm not sure if this is solvable with an app to be honest. And, you will be competing against listings with attractive-looking properties that aren't actually on the market, so on the surface-level won't look as good. Good luck!
I think a lot of the issues would be solved by moving the power from the agents and lettings management services to the tenants and landlords. OpenRent is already trying to do this, and I've had good experiences with them so far.