I've taken a street portrait photo (here) that I'd like to enter in a competition (the Sony World Photo Awards). I asked the man in the shop if I could take his photo, and he was more than happy for me to do so. I never asked him to sign any model-release document as it has very much an "off the moment" photo.
The rules of the competition state:
You confirm that each person depicted in the Entry has granted permission to be portrayed as shown.
and also says that:
[does not] violate any person’s rights of privacy or publicity
With this in mind, would it be valid to enter the photo? I'm sure I've been to exhibitions before which include a lot of impromptu portraits and street photography where I'd imagine that the photographer didn't go running after the subject with legal documents, but I could be wrong...!
(if it helps, I'm based in the UK)