Since you don't seem to have been anywhere near a court, I assume that you've actually received a 'fixed penalty notice' rather than a 'fine'.
FPNs are not the same as criminal convictions. You don't have to declare them if an employment application asks about convictions. Similarly, they won't show up on a CRB check at standard level. A CRB check at enhanced level (such as that required for jobs working in schools, etc) won't automatically show FPNs but they can show any information (such as FPNs or even unsubstantiated allegations) that a senior police officer (or other appropriate authority) regards as relevant.
Chris