Well when G-YMMM was w/o BA was left with only one spare covering LHR/LGW (based from LHR) 772 operations. Cost is irrelevant, the long-term impact of having an operation run ragged isn't cost efficient in the long-run.
What @Machinbird says, Boeing will Material test the aircraft. G-VIIO had a D Check in Cardiff last year, and B check end of last Year. According to The BA Source. But the problem isn't the wingbox, it's the wing-spar that was exposed in this case.