Has anyone managed to use a UK upgrade certificate to upgrade from a booking class that isn't allowed in the small print? I was wondering if they always follow the letter of the law or are they ever flexible on this.

I'm a BD gold and I'm travelling on TG in business but my ticket is in I class not C or D. Is it worth saving a couple of certificates for my trip LHR - BKK in January just in case or should I go ahead and use them now on a short European hop?

Thanks for your help.