IMO, you're mixing things up a bit.
Copyright belongs to the original creator (except in a few instances). As long as the photograph is not of a copyrighted piece you own the copyright.
The second question becomes about "restrictions to copyright." What you can do with the picture you own copyright to. Part of this question is was the image obtained "legally." Having obtained an image illegally does not transfer the copyright to someone else, it just means you have no rights to use it in any way (you could be forced to destroy the image to ensure no usage as it was obtained illegally).
"Legally" obtaining an image generally means you obtained it in accordance with any restrictions placed on taking the picture in the first place; via an individual's or organization's privacy rights. (Civil law is still law).
If it is a "private function," paid or not, then the private entity putting on that function has the right to restrict photography as they see fit.
If it is on private property, entry controlled or not, then the private owner has the right to restrict photography as they see fit (in most situations/countries, but not all).
And if it's on public (government) property there may be restrictions to photography... usually to "professional/commercial" photography and due to "safety" (i.e. setting up a bunch of lighting equipment). But it may be due to other reasons. All land is under someone's control/authority...(in these cases a violation could be "criminal" instead of "civil")
In many cases these restriction do not have to be "known" by the photographer, they just have to exist. If you enter onto private property because you did not know whether it was private or public; then you are trespassing and subject to the landowners restrictions (in most cases/countries. I *believe* this applies to the U.K. in general). Ignorance is not a defense. But they do have to exist prior. For example you enter onto private property with permission and without restriction, and start taking pictures. Then the property owner says "don't take pictures of my family." From the time of notice you have to comply.
So the next question is did TOU's exist, not did you know of them. Were they on the organizer's/property owner's website? Any paperwork that may have been distributed at the event? When you are going to do something it is your job to ensure it's "legal;" it is not their job to ensure you know.
Assuming the photograph is not a derivative work of some other copyrighted material then you own the copyright. And assuming there were no restrictions in existence at the time the pictures were taken, then they were taken "legally" and can be used as you see fit. This excludes the use of the photograph of a rider for "commercial use" (advertising) as that may violate the privacy rights of the individual (unless otherwise waived).
But what you "have the right to do" and what "you should do" may be two different things. For example, you take a families portraits and they signed a waiver. After the fact they ask you not to put their pictures on the web. You have the right to ignore their request, but doing so would probably be a very bad business decision....that's for you to decide.
So, how much is a facebook post worth to you, and what's it likely to cost you?
***there may be minor technical variances to what I've stated in various countries/states/situations.
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