HMRC classes any car that is at least 20 years old and is worth at least £15,000 as officially ‘classic’ or ‘heritage’.
But unfortunately, there are no such specific criteria when it comes to insurance companies’ classification.
You will have to check with the specific provider in order to see whether or not they see your vehicle as a classic, and thus what kind of policy they can offer, outside of those available to drivers of conventional vehicles.