This thread should be titled "BA aircraft in entirley normal landing".
Reductions in declared distances happen all the time, as do temporary changes to obstacles as has been pointed out in this thread.
Remember the 777 that bellied out at LHR? That runway remained open, albeit to departing traffic only, with reduced TORA/ASDA.
The real unknown in this conversation is what happened to the disabled aircraft? Had their mishap only just happened? And critically, did this have an effect on the level of fire cover available?
You might have all the runway you need still available, and ATC might allow you to go for it, but if the fire cover is diminished by an incident in progress then you wouldn't be legal.