I have a case for UK261 Invol offload LHR-BCN, but it's sufficiently bizarre that I thought I would run it by this crowd first.

I had an Avios ticket for the 12th of June
On the 8th of June I went to change it - I would normally cancel and create a new one, but I noticed there was now a "change" option which looked easier. How wrong I was. I changed the ticket to the 13th of June. I got a confirmation email of the new flight
My app updated to the 13th of June flight and I picked myself a seat
Check-in opened as planned, but I wasn't able to check-in online. This always makes me nervous, so I checked the ticket was active on MBB, showing the right flught/ date and that there was a ticket number which there was
I get to the airport and I'm told that I cancelled the ticket on the 8th of June and so can't fly
I spent sometime with the BA Gold line and what they're alleging is that within seconds of me carefully changing my flight, I then asked for it to be cancelled and then forgot I did it. Coincidentally, the refund then went very badly wrong and so it was never processed and so the ticket appeared to be valid and actually no refund had been made
Their view was that BA had no liability for this situation and so they would now process my cancellation on the 13th of June but that I didn't have a valid ticket, there were no more Avios flights available and the only option for the seat was to pay £666 for a single from LHR to BCN. They also take the view that because I apparently wanted to cancel the ticket (as unlikely as it sounds), the ticket was no longer valid even though it appeared to be and no refund had been processed

Any ideas? The cancellation process takes 2 clciks, so I'm pretty sure that I didn't actually do it - but it's hard to explain just what happened here. The agent on the Gold line was friendly, reasonable and helpful, but BA overall didn't exactly come out smelling like the world's favourite airline. The BA First check-in agent who insisted that British Airways had no connection to the BA Gold Line was particularly bad at customer service.