Hi andrewberesford,
If you wish to port your number from Virgin Media to a different Operator, you will need to ask them to submit a port request to us. @BenMcr is correct in that if you currently have a bundle, your billing may change with the landline service removed.
To port, the line must also be active, so you cannot downgrade and then port. Instead, it will happen at the same time. The downgrade will not affect your broadband services as mentioned above.
Regards,
Lisa