Just to give an update about this, I called the BAEC and gave the agent the reference saying that the ticket was issued on 125 stock. The agent checked with the back office, having put me on hold for a few minutes, and came back to say that the ticket should indeed have a free bag allowance (for 20kg). So he said he would reissue the ticket. Checking on Flybe.com I see that 20kg has been added to the booking.

Edit: just to say that they rang a moment ago to say that in fact a free bag is NOT included in the booking. Confusing, or not?