Currently have:
LHR - SCL biz (used Lloyds upgrade voucher for two, applicable Avios and taxes)
SCL - LHR eco (that part is just paid out normally, full Avios and taxes)

I've seen an opportunity to fly back from GIG - LHR instead, in a better cabin. I assumed that would be just a change fee (£35 each).

However, agent is saying I can't use the voucher on the return. As I see it, it hasn't been used on the return, only on the outbound. Apparently it's all one ticket and it's not possible to cancel return leg only of ticket either.

Sole option I've been given is to no-show the SCL - LHR and purchase the GIG - LHR separately. Which I won't be doing. Seems a bit harsh?

[I can't figure out where else to post this beyond British Airways forum]