you can migrate from datastream to IP stream - I do believe BT released a process at the end of may for widescale migrations. Looks like Tesco may be supplied by Tiscali. As far as I know tiscali are the only major datstream supplier left since Telefonica bit the dust that combined with teh no MAC policy
Tesco Internet Access do not provide any MAC codes unless there's a very good reason for it (service issues or service complaints procedure).
Single-User migrations from Datastream to IPstream is very very very rare; they're only ever done in mass migrations.
To be honest with you, the quickest way to get this done is to cancel with Tesco, wait for the ADSL cease to be applied, and re-apply with plus.net. When they do dish them out, Tesco take their time with the MAC requests