Ok Dedworth now you have presented a straightforward situation. Had your friends attacked the demonstrators they would have committed an offence.
Not so unreasonable when you think about it.
It would be quite wrong if I were allowed to break the law because I believe the UK had no right to invade the sovereign countries of Iraq and Afghanistan. Our boys should not be there, in my opinion, but that does not give me any right to attack those who think otherwise.