I am familiar with Skyscanner.
I used to use those price comparison last minute deal types of sites but just dont bother anymore as i found them a bit of a waste of time. If the usual carriers need to fill seats at the last minute they'll list the prices on their own repective websites.
Warsaw - Uk you have: easyjet, wizz air, ryanair. The usual suspects.
I have never found good prices with Ryanair. They never have anything when and where I want.
yeah its not very often that they have last minute deals as theyre so good at filling up their planes, it does happen though, worth a look.