What happens once I have my MAC?
If you give your MAC Code to another ISP, your Broadband service should switch to your new provider within 5 working days. In this situation, your 30 day notice period will commence on the date that the MAC was issued, and this will be reflected on your final invoice.

Looks like this coming bill will be the last one.

That sounds as if the account will be closed after the 30 days from MAC issue, and relevant payment/refund payments dealt with.

It's still worth confirming with them that they know you have migrated away. I've migrated a few times, and the losing ISP has always known, so I didn't need to tell them, but we sometimes see people here where things have gone wrong and the account stays open and charging.