As a data point, I had to request a receipt for a change fee recently. In this particular situation, in the original itinerary I'd UUA the inbound to First. In the end, I needed to change the inbound to depart from a different location on a different date (lost the upgrade unfortunately). What I was getting when I requested a receipt was the tax fee I paid when I originally upgraded and *not* the fees I paid to change the itinerary.

I submitted a request to support and they responded saying they could not give me a proper receipt for the change fees due to IT limitations (typical). What they did provide me with was a letter (on BA letterhead) stating that I'd made a change and paid £xxx to do so. Luckily my employer was fine with this, but I wanted to post this data point to warn you that you may get the same.