What I found to be truly nightmarish when looking for a flat was:
1) The immense majority of flats listed on rental sites (either aggregators or the letting agencies sites) are either scams, have already been rented or are just baits that letting agencies publish just to get you to ring them so that they can try to sell you the flats they actually have available.
The result if that pretty much all the online rental listing websites and apps are nothing more than a spectacular waste of time as every time you find something that looks decent, it's either already gone, it's a scam or it was never available in the first place.
2) In a place like London, the rent is only one part of what a flat costs you. Council tax, charges and agency fees are massive and can easily add 20%-40% to the cost of your flat. And these charges can vary a lot from one place to another. Yet, no site ever displays the total cost of a flat. This makes it really hard to compare flats.
To address the first point, I always thought that charging both landlords to advertise and tenant to look for flats would cut a lot of the crap. Landlords should only be able to advertise for a maximum of 3 days (they could pay again to extend for another 3 days). Since tenants have to pay to see the listings, landlords shouldn't get harassed by time wasters. Having to pay to advertise should hopefully keep the scammers at bay. And since landlords have to make the effort to renew and pay every 3 days, it should hopefully reduce the amount of flat still listed but already gone.
Of course, it's likely that scammers and letting agencies would quickly find a way around this - but it'd be worth trying. Might not be your space right now since you seem to be focusing on aggregating from existing sites. But if you want to add real value, I really think you need to re-think the way rental sites operate.
(as a side note, the only rental site in London I've found not to be a big fat waste of time is http://london.spareroom.co.uk/ - pay for access to the latest listings and you generally find genuine and good offers that are still available. And it's not just rooms in shared houses - there are actually quite a lot of flats to rent there as well)