Can I change the name for the main policy holder to mine, I am the named driver?

If the VC5 document is in your name which means you're the registered keeper, then you would be able to change the main driver to yourself without any problems. However your insurance premiums may change as a result. For example, it could increase if you have less driving experience or no claims bonus than your ex-partner.

You should also be aware that your insurer may charge you an administration fee to edit the policy documents and send it back to you.