A motoring offence dealt with by a fixed penalty is not a 'conviction' and therefore never needs to be mentioned if only 'convictions' are asked about. An offence resulting in a conviction before a court needs to be declared only until such time as it becomes 'spent'.
Once a conviction has become spent, the provision of the Rehabilitation of Offenders Act allow you to lie and state that you've never been convicted of anything. (There are exceptions when applying for some types of employment or voluntary work but they don't relate to insurance applications).
'Ancillary matters' are also covered. For example if you were declined insurance solely because of a criminal conviction but then later (after the conviction became spent) asked 'Have you ever been declined insurance cover?, you could lawfully answer 'No'.
Chris