I forgot, it depends on your current ADSL service. There are 2 types, I think they are standard ADSL and MAX ADSL(which uses the LLU).
I was on MAX ADSL (which is the newer of the 2 types) and because Sky were not on teh LLU migration trial, Plusnet said the MAC address would not be transferrable, so I had to disconnect from Plusnet before I could order from Sky.
Simply disconnecting from Plusnet took 3 weeks (lots of e-mails, telephone calls and postings in their public forum grr).
If you are on the standard ADSL service (speeds of up to 2MB I think) then you should be able to migrate using a MAC address supplied by Plusnet with little/no loss of service.
<Plusnet are rubbish!!!>