Okay, you have a few options.
Firstly, in the report you could make clear that you weren't fighting Bretonnians background-wise. Perhaps these were marauding bandits? (So you know, it was suggested that Bretonnian players fighting Empire armies should describe them as, for example, Marienburger mercenaries, to avoid other problems.) Or you could simply say you were fighting rogue knights. Not all knights of Bretonnia are honourable and we do make provision for renegades.
Or secondly, why not say it was a training battle? Imperial and Bretonnian soldiers meet, and they go through some exercises to hone their skills before facing the forces of evil.
Or perhaps due to circumstances you did have to fight, but in such a case I recommend including the sword-cleaning clause, which apologises and absolves the commander of any guilt.