Contrary to Undercover's post, the Rehabilitation of Offenders Act applies to any questions (either written or verbal) that you might be asked when seeking to enter into any contract (whether that be a contract of employment or an insurance agreement). The only times when you can be required to disclose 'spent' criminal convictions (or be deemed to be in breach of contract for failing to do so) are when the circumstances are exempt from the Act (e.g. when applying for employment within the 'caring professions').
You can answer 'no' to the question without worrying about invalidating your insurance. Note, however, that if the insurance company is aware of your conviction by some other means (e.g. if you declared it on an earlier application for insurance before the conviction was spent) they're not obliged to ignore it and they would still be entitled to refuse you cover or charge higher premiums.
Chris