Jonnyh
It would be simple to just to wait that 30mins for the BFS/brs plane to arrive however the problem is crew.
The crew will not work a further 3+hrs to Amsterdam and back and they may have already worked 10+ before as the crew may have worked say ncl-alc-ncl-brs
They may be close to being out of hrs or be timed out for tomorrow working causing further delay
It's easier for the company to use the spare crew and aircraft at which time they were using gatwick crew