We do not cut your service off until BT have confirmed that you have migrated.
As such, you will almost certainly be on your new service before we are informed of this though.
What can happen though, is that BT prevent access to the PlusNet network from your line as part of the migration process (though they often allow a few days for changeover or forget to do this at all).
Requesting a MAC does indeed provide your 30 days notice. You don't have to use the MAC though, and should it expire, your service will continue as normal and you would need to submit a new request.