I've done this a couple times at Gatwick.

Last year I picked up a £110 roundtrip with Adria to LJU.

Most recently about 3 weeks ago, I managed to pick up a £125 roundtrip to VIE outbound on Aer Lingus and back on Easyjet.

I did book the flights online from my laptop though once I looked at the departure screens and found an interesting possibility.