I believe this:
The Entrant warrants that any person depicted in their photographic entry, has granted permission to be portrayed in the photograph.
is not of any consequence, since (as we've all learned from numerous discussions and submissions about photographers' rights over the last few years) you do not need people's permission for them to be portrayed in photographs. What you do need their consent for is commercial use of their image. If a photograph simply enters and wins a photography competition then that doesn't count as commercial use. If you (or the BDO organisers you signed your rights over to) wanted to use the photograph to promote the event or any other merchandise then that would be commercial use and require
model releases.
However, this purpose is merely hypothetical to you as the photographer and competition participant because BDO have not stated that they will use the photographs commercially. They've only asked you to sign over the copyright to them. It should be up to them, at the point in time they make the decision to use the photograph commercially, to obtain the necessary permissions.
In short, this sounds like trouble, I'd give it a wide berth
Cheers
Steffen.