This has nothing to do with BA and everything to do with the clear record that you have made here on FT to the effect that you knew, at the time you booked, that it was a mistake fare.
No matter what the situation is for the other routes you cite and the fact that there may be dirt cheap fares to TOM (Timbuktu in case you aren't intimately familiar with IATA codes!), in this particular situation, you took a no cost gamble and did not succeed.