The AIB report does not seem to make any comment with repect to the conspicuity of the Midland aircraft on the runway other than the fact that strobes were not switched on.
I wonder whether a different colour scheme would have made the aircraft more conspicuous?
Having read the report I think it is quite understandable that the BA crew were under the impression that the other departing LH traffic was the traffic that was relevant to them. We should also remember that BA crews fly worldwide to other destinations which are just as busy as LHR, etc.
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