I went through all this when I migrated from Virgin ADSL to BT FTTC. Virgin gave me nothing but grief and tried (but failed) to charge me but they are NOT allowed to do this according to OFCOM. The latest OFCOM rules are shown below (see site maccode.org.uk for further details). But do not confuse the issue of issuing a MAC with the notice period that your current ISP may be entitled to. They are totally different issues and involve seperate financial obligations.
New Rules by Ofcom on switching
With the introduction of new rules, OFCOM has curtailed the rogue practices by some ISPs who effectively prevented customers from migrating to new ISPs by not issuing MAC code. The following are the salient points of the new rules:
1 The existing broadband provider must supply a MAC on request by the customer within five working days, along with its expiry date, and must not charge for this service
2 The new provider must accept a valid MAC.
3 The existing provider cannot withhold a MAC for any reason like debt collection, or for contractual reasons
4 If the customer requests for a MAC inside the minimum contract period the customer will still be liable for any contractual obligations (including paying subscription charges for the minimum contract period, etc)
5 The provider must explain the reasons to the customer if a MAC cannot be issued.
6 Providers should offer the customer reasonable assistance in migrating from or to another provider
7 The provider must not disconnect a customer’s service if a MAC is requested
8 The provider must provide two different ways to obtain a MAC (e.g. by phone, post, website, etc)
9 A provider must re-issue a MAC on subsequent requests if the previous MAC has expired
10 The new provider can still refuse to supply a service for business reasons including credit worthiness
11 It is only compulsory for an ISP to provide a MAC for home consumers and small businesses (with fewer than ten employees).