This seems like a tremendous own goal. They have found a formula which - for a limited time - people will pay above the odds to fly on a 747 in a BA retro livery on domestic routes but have cancelled it!
If operational delivery was the issue, a programme which committed two of the three 747-400s to each doing two domestic round trips from LHR each that day - and perhaps the A319 too - would surely have been deliverable with the third 744 in the fleet as flexibility to cope with any disruption issues.
As it is, they’ve lost revenue and disappointed people. Just what you surely would not want ?