I’m not sure what’s so unbelievable. A quick search shows £15 return (yes, including taxes, charges, APD) from STN-WMI for most of July. 5-8 July is an example. Or £13 return to Krakow on the same dates. The Polish and Eastern Europeans are happy with a middle seat, a tiny bag crammed under their seat and eating their own home made sandwiches, Ryanair aren’t making any money on auxiliaries on these flights either.
£13 is APD. Hence it’s dumping capacity to prevent competitors from operating. Ryanair have the financial might and cheaper cost base to do this, easyJet and others don’t. Which is why EZY are being much more conservative than Ryanair.