Different markets. You are looking at GLA-MCO, which is not the same market as GLA-LGW + LGW-MCO.
You have simply hit the same issue thousands of passengers hit every day, the fact that it may be cheaper to purchase separate tickets. But, separate tickets come with the risk that you are not connecting. If you no show for a segment, the remainder of the ticket is cancelled. Thus, one must allow sufficient time at the transfer point, e.g. LGW to avoid reduce the financial risk and to deal with the fact that one would have to claim any checked luggage at LGW and lug it to BA check-in for the MCO flight.