If you are a "no show" for the outbound, the remainder of the reservation will be cancelled. Your ticket will retain whatever value the fare rules for that fare basis provide. On a fairly typical penalty fare basis, this means that the fare retained is zero and you may recover taxes less an administrative fee. That will be fairly close to nothing.

More important, your Wednesday return will have been cancelled and according to your research are sold out meaning that you will have to hope that there becomes availability and you can be assured that the new availability will be at fulll price.

BA will not protect you. Ordinarily and dependent on where you are, it would take a very kind agent to restore your ticket. As flights are sold out, that may not even be possible. You certainly will not know until Wednesday.

It may cost more than the single to make the changes, but in the end, you generally need to look at the overall cost of the journey as you wish to schedule it.