This is a good point. I just looked through my profile and couldn't find a unique number. There is an 'old membership number' set at 2498.
https://www.rtoc.org/boards/member.php?u=60
Then I looked on the reverse of my latest membership card and the number appears there too. It doesn't look like you have a number in your profile but maybe you do on your card?
If you can find these things, then the way to prove it to your insurance company is to provide your login name, number if you have one, and then the contact email ansd URL of the club so that they can check if necessary.