I get that, but at the same time before I buy anywhere I'd view dozens of properties. So the chance of me buying any one in particular is low. So I don't think the "getting their hopes up" argument is valid.
I do know that I could afford the flat, even if I can't prove it; I have more than 10% deposit, the list price is significantly under my upper borrowing limit from online mortgage calculators, my experian credit score is 975/999, I have no dependents so my outgoings are low.
The property is vacant so it's not like I'll be inconveniencing the owners, only the estate agent (and that's their job).
Also the property has been on the market since December 2015. If it was highly sought after then I'd understand, but clearly they're struggling to sell it (it's right next to the railway lines and the lease will need renewing in the not too distant future) so I can't believe they're being so picky.
Personally I just think it's terrible customer service, and to be honest what I've come to expect from estate agents and letting agents.