No reason for the cancellation given by BA. Agree that it seems mad not to offer a codeshare flight to avoid compensation payments, plus additional claims such as hotels, meals, etc. Total cost to BA must be more than moving someone onto a codeshare service.

This was booked via my corporate TA, so no chance to fall back on my Amex card.

From looking at the arrivals list into LHR yesterday, there seemed to be a number of cancellations (certainly seemed more than normal), so maybe something to do with their computer system failure that morning? Would that be "beyond their control"?