Being issued an FPN by police or any other public authority (e.g. - PCN) is not a criminal conviction. A court issued fine may be - but that is also subject the nature of the offence and circumstances of how the matter is dealt with in the court.
Basic principle - if there is no court involved there is no criminal conviction or civil judgement.What is a Fixed Penalty Notice?
1. The FPN is not a fine.
2. The FPN simply discharges your liability to conviction for which the notice was issued.
3. The FPN is not an admission of guilt, it only removes the possibility of you receiving a record of Criminal Conviction.
4. Receiving a penalty notice does not mean that you have received a criminal conviction. The FPN is given instead of a prosecution in court.
5. If you have received an FPN, you have 14 calendar days to pay the penalty or to request a hearing.
6. Failure to pay a penalty may result in a higher fine imposed by the court or imprisonment.
But if convicted by court, then depending on the offence you may have an unspent conviction. If you wish to, you can check for unspent convictions on your own.
Endorsements and penalty points must stay on your driving licence for four or eleven years depending on the offence. But just an endorsement on a driving licence or a driving disqualification doesn't affect the result of a Tier 1 (General) immigration application.
Under current rules, unspent criminal convictions or civil judgements have no effect on Tier 1 (General) extension application - neither for the principal migrants (245CA) nor for any dependants (319C and 319H) ... even though the form asks the applicant to disclose "unspent criminal conviction(s)".
Also, it is better to declare an offence (specially so if you are unsure if it has led to a criminal conviction or not) than to not disclose it.