A fixed penalty isn't a conviction. A conviction only occurs if someone appears before a court. If someone is asked if they have any driving convictions (and they've only had fixed penalties) they can lawfully answer 'No'.
Of course, if the form asks for details of fixed penalties as well, they must be declared. The form should have a separate section for that but, if not, the only option would be to enter the date of the offence (as shown on a driving licence) as the [nominal] date of conviction.