BA upgrade voucher on JAL?

[wily_coyote]

Hopefully an easy question! I’m flying with Japan Air from Heathrow to Tokyo. Can I use an upgrade voucher with this flight? Or is it limited to BA flights only?

I booked it through Finnair website, which perhaps complicates it further!

Thanks in advance.

[Froggee]

BA only I’m afraid (assuming it’s a BA upgrade voucher!)