There is still availability on this flight at T-6 & I've just got off the phone to BA & they could only tell me that online checkin has not opened for that flight.
I asked why this may be but they didn't know.
One of the suggestions given to me was that there had been problems with BA.com recently & that maybe this could be the reason.
I was then told that there were no changes to that flight & to go to the airport as normal & checkin there.

SJ