This is a bit different. The issue is not that the delay was occasioned upstream, but rather that the delay was occasioned by the failure of a part which had been properly maintained and was not scheduled for replacement.
Consistent with at least one non-binding report to the EC, that constitutes an "extraordinary circumstance" and accordingly EC 261/2004 compensation for delay would not be payable.
To be clear, under this interpretation, had OP been a pax on the AUA-LGW and had that flight been delayed by 4+ hours as it likely was, BA also would not have paid compensation. The theory is that the carrier, BA in this case, has done all that it can do to assure an airworthy aircraft and that therefore the part failure is not within its direct control.
Will be interested to see how this turns out.