You might have been cut off, yes, but your phone line is still physically attached to Freeserve, which is why you can't register for another broadband ISP at the moment.
Basically you can do one of two things:
1) Completely cancel Freeserve and get their service disconnected from your phone line. This frees it up and you can now pick a new ISP.
2) Ask for that server code or whatever they're calling it they're asking for from your old ISP. Basically it's just a reference code for your ADSL connection to let you migrate from one ISP to another - the ISPs can use this and directly transfer the connection from one ISP to another, rather than having the line disconnected, then you wait 15 days or whatever just for it to be reconnected again for the other ISP to use. Make it clear that you want to migrate or change ISP so you get the right code, and make sure it's the old ISP you're calling to get the code from.
In either case, you'll no longer be able to connect to your old ISP. Also, I'd suggest you read the Terms and Conditions of whoever you pick as the new ISP and what limits there are if any, otherwise you might just end up doing the same thing on the new ISP
