The only way to answer this is to carefully read the fare rules. Not just the key portions at the top dealing with change fees, but the endless restrictions which may apply to the best fare available today. Perhaps there is some odd married segment, fare restriction or other oddity to what BA is selling today for that specific route as opposed to what OP purchased.

The fact that someone else is able to book GLA-LGW-Carribean has nothing to do with another filed fare on INV-LHR-US.