I used a pair travelling from Hong Kong to Osaka. I assume this was the first time they had been seen since the check-in girl called her supervisor, but they allowed the upgrade even though the ticket was not issued in one of the fare classes mentioned in the Ts&Cs on the back.
Flight was reasonably empty so I don't think they had any difficulty.
I am certain that your highest chance of a successful upgrade will be to use them where there are not likely to be other Brits using them at the same time.
Nick