First of all, on this website, you never need to worry about
OK. What was on the form that you signed? Did they give you a copy of it? Did it say anything about
you accepting your guilt and/or that it constitutes a conviction? You say you were fined - did you actually pay the fine, or did you opt to be heard by a court because you disputed your guilt and/or the fine?
If you didn't sign anything that accepted your guilt and you have not been to court or been contacted by a court about
conviction in your absence, then I can't see how you can have a conviction. But did you perhaps move address shortly after this happened, or is there any way in which there may have been difficulties contacting you in the weeks and months after the event?
To be absolutely sure of knowing whether you have a conviction, you can do a Subject Access Request (very precise information here: https://ico.org.uk/for-the-public/personal-information). Address it to the police force of the area you were in when you got stopped/fined. By law, they have to respond to your request within 40 days and it must tell you all the information they have on you.
Good luck and I hope you can visit the UK again.