I would have thought that, if they had had time to have one made for them, seconded British officers would generally have worn their Portuguese uniforms in battle, if only to inspire their men. Beresford certainly wore his, IIRC, but then his posting was long-term, whereas the men Wellington tended to zig-zag between the two armies were usually very competent types who were invariably without wealth or patronage, but deserved a "leg up" (he was far from the miserable curmudgeon with junior officers that he is often made out to be).