I don’t think the op can hope for much else, and I personally believe it is a fair outcome too. BA was not at fault in any way and they do not really owe anything here (they could have been meaner) so a voucher to use on BA in next 18 months is far from unfair.

If that helps you could ask if you could get the voucher value as BA Avios instead. That is the only alternative which just might be accepted and can be used to purchase award tickets on UL, QR or AA for instance, but that only really makes sense if you already have a BAEC account (and no they won’t give miles on other airlines programmes nor cash, why should they?)