In the past you needed to get a Migration Authorisation Code (MAC) from your current provider and give it to your new broadband provider.
You no longer need to do this if your current provider is on the Openreach network e.g. BT, Sky, Plusnet, TalkTalk, EE, John Lewis, etc. Your new provider will handle the switch for you.
If you are switching from a broadband provider that runs their own network like VirginMedia, you will need to contact them to cancel.
TV services/bundles also need to be cancelled directly.