Do you mean campervan insurance, similar to the insurance that you would have on your car, or do you mean holiday insurance similar to what you have when going abroad?
If you are hiring a campervan it will presumably include insurance & also breakdown insurance, yes? You don't need travel insurance in the UK, we have the nhs.
Holiday insurance is not just about covering medical treatment, it also covers cancellation due to illness, redundancy etc, and personal belongings etc. You may of course be covered by existing policies, best advice would be to check this out.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.