Hmm, bummer - I used JFK19 for ~33% off an anytime return purchased in early January about 3 wks before travel. Ticket was £24.40 and I used (the very underwhelming) First Class due to Star Gold status.

Of course my QR code wouldn’t work at T2/3 in either direction (worked fine at Paddington) but both times they let me through. And my return ticket was disabled on the iOS app after I took the outbound from the airport. Apparently a known glitch, but fortunately it was easy to get through to the call center and they emailed me a pdf.