You can downgrade to broadband only if that is all you use, & it will halve your bill if you take the lowest tier. Question is are you out of contract. If you have not made any changes to your services in the last 12 months you should be in the clear.
The one problem you have is that you have to use the phone to downgrade services, as Retentions, who you need to talk too, are only available via this means. One way you can get round call charges if you have a smart phone is to install the Smartcall app & link it to your landline account. You can then phone VM on 150 for free no matter who your mobile operator is.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV+), Talk Unlim Telco, VIVID 100, Virgin PAYG Mobile