I tried to use a £35 Internations voucher ex-UK but received the following message:
Please review the fields below to continue :
• One or several airlines are not supported by the voucher submitted. Please check the voucher conditions or continue without voucher.
The return leg is on a LH flight operated by BMI on wet lease. With 3 out of 4 daily MAN-FRA flights and 2 out of 3 MAN-MUC operated by BMI, this rather limits the usefulness of Internations vouchers.