I found this out today (about 4 years too late) that even though the Lloyds Upgrade Voucher can not be used on BA Cityflyer, the Lloyds Economy Companion voucher is valid on these.

I used this on my flight to Ibiza and got a decent value.

I understand that there will not be many people who still has the Lloyds Premier Avios Credit card but hopefully someone else will also find this info useful.