Fair enough.
I think BA are allowed to oversell F by a maximum of 1 on most routes. Club is a different story, but I am aware of it being oversold by up to 10 on occasion.
I appreciate that in F you should expect a guaranteed seat but you should also expect flexibility. The challenge of the airline is to manage these two contradictory (from its perspective) expectations so that it can run a profitable service. BA have obviously made the calculation thus
number of flights per day..........large number
number of flights where F is fully sold....small number
number of flights where F is oversold...tiny number
number of flights where F is oversold and everyone turns up...miniscule number
and therefore the loss of business arising from the final scenario is tiny. This could then be set against the potential loss of business from effectively not filling the F cabin on a regular basis.