Based on Revenue Management theory:

If an airline has 100 seats and only ever sells 75 seats at, on average, £100 then by reducing the number of available seats to 80 does not mean that the market is prepared, on average, to pay £110 for those seats and therefore the prices remain as they were.

However if they reduce the number of seats to 50 then the airline should remove the 25 seats of the lowest fares normally bought in order to optimise revenue.

Therefore I guess it depends on which scenario BA's situation falls into.