What could the police do about it if the million BNP voters were to fly a Union flag at the same time ?

Well, although I am a member of the BNP, I don't see the Union Flag as belonging to them. On the other hand I don't have any objection to *any* British Citizen flying it, or the St George's Cross for that matter. I'll go even further, and say it should be a criminal offence to burn or otherwise deface one.