We too suffered a cancellation of the BA 755 back in December - when Heathrow was affected by strong winds. Obviously (according to BA) this meant that the cancellation was due to weather, not operational reasons. I didn't bother pursuing a claim.
However, it appears that BA's policy is to cancel SH not LH flights when heathrow capacity is limited. Maybe BA755 is one of the first to get cancelled in such circumstances.
Does anybody have a list of the flights most likely to be cancelled for operational/commercial reasons when Heathrow capacity is reduced? I'm sure BA must have such a thing -a sort of priority list of flights.