Originally Posted by
corporate-wage-slave
If the BA service is cancelled by BA then yes you can normally be moved to SOU, that's well within the 300 mile gateway change rule. However as things stand it is only per sector affected by the cancellation, so you can't move the return flight if that still operates to LGW. It may be an academic concern, in that if you wait you will find both LGW sectors get moved to LHR anyway, so as with many things in this space, it's best not to change at the first moment but to wait and see what happens.
Very helpful cws (as usual), thank you. I may have booked ex-LGW flights entirely speculatively (expecting some sort of cancellation) with the backup option of cancellation for an eVoucher. Used eVouchers for the payment so the actual cost of this is about 0 with a tremendous upside.