You are correct it isn't a conviction.

However they are asking you a direct question 'have you been on a speed awareness course', which if you did not answer truthfully, you are obtaining the price of your policy by deception. Technically fraud. They would be well within their rights to cancel the policy.

This was all over the news a few weeks ago, and is the accepted position.

If they do not ask you, you do not have to tell them, as it isn't a conviction. But if they ask, you MUST tell.