As a match goer who personally had 4 made last year ...2 at 8ft by 5, 1 at 8ft by 4 and 1 at 6ft by 3 at great cost, I can tell you that proper flags are made.
In London nowadays there is alot of red tape with councils and with Chelsea and Arsenal you have to now have proof of a fire certificate if asked by a steward. In certain areas of the stadiums there are limits on size of flags plus if they cause viewing restrictions they may be asked to be not flown.
With regards to the plastic ones seen at grounds such as Stamford Bridge, Wembley for big games....a lot of clubs do it including Utd, Arsenal, City etc.
My own take on it is that if you dont like them then give it to a kid or someone who does...I always pass mine on.