Obviously the pax on that LHR-LIS flight will never know the reason, but if I ever found out that my flight had been delayed because of a shenanigan like this, I'd be pretty annoyed.

In my earlier post, I warned about intentionally abandoning the bag at LHR, hoping that PPBM would catch it and cause the bag to get offloaded and conveniently reunited with you right there. I was thinking this was an AA tech stop (same metal on to LIS) and they'd be pretty pissed off at disappearing pax - especially if there was a note in the record that the pax had been asking about short-checking in NYC to begin with.

Of course, this is an interline, and BA would never know what the pax discussed with AA in New York. But still...

You've found a cheap LIS-LON segment: seems like buying that is the least risky and most honest of all. Think of the $81 as a small bit of insurance against something going rather wonky during this excursion.