I have found that it is possibly better to cancel your contract and request a total cease order (via BT Wholsale) by phone contact and backed up in writing than trying the MAC route.

Where did you find that? You need to be desperate to go that route. New OFCOM regs re MACs came in on 14 Feb so should be easier.
A MAC migration gives minimum (30 mins?) loss of service, whereas cease and reprovide can be 2 weeks or more, particularly if Pipex are slow to get the tag on your line removed.