I'm not sure they need a warrant. As I understand it there is nothing to stop me walking into your garden. If the piping is their property I would have thought they have the right to disconnect it.
They will probably argue that this was covered by their terms and conditions and you had effectively agreed to these by accepting the service, and you may well have had leaflets/bills containing details of their terms.
Having said that, I'm surprised they haven't tried to resolve the situation first- have they never responded to your bill query?