From BA bidline rules
BLR 10.1 "British Airways will plan Trips within Section 10 limits; these are the limits of
the Pilot's contractual obligation."
(industrial)
It then says
British Airways may request and anticipate that the crew member will be prepared to extend to the relevant Flight Time Limitations of the British Airways Scheme. BA crew management recognises and accepts that there will be wholly proper reasons why, on occasion, an individual crew member will be unable to accede to this request. Such reasons would include (but need not be restricted to) the pre-flight rest obtained, previous fatiguing Duties, the displacement of the next Trip or social commitments of an important nature."
ie any reason you can think of.
Therefore you can refuse to extend Industrial limits up to even FLTs(never mind discretion on that) and BALPA have publicly stated that they will fully support any pilot who refuses for whatever reason. Can't be any clearer than that.
Thankfully for BA this rarely happens.
BTSM