Seems reasonable to me that if you want to do a race that someone else has been kind enough to organise, you respect their rules. If you don't like 'em, do summat else.
My only appeal to organisers would be NOT to produce massive flag-like numbers. I'm quite small and have restricted chest space for advertising matter.
Fell races are usually OK in this respect; road races can be difficult if the number is larger than your vest.