If you are getting BT bills for the line then it is already active!! The fact you don't use it is irrelevant.
Or do you mean you have moved from that address and don't have a phone service at the new one?
As for tying the rental and broadband onto one supplier that is often a recipe for poor internet. You can switch your rental and call billing to someone like the Post Office, cheaper than BT, but their (newish) broadband doesn't get good reports here.
Any good ISP will still regard it as a BT line as that is actually/physically what it is, BT Openreach.
What are your broadband requirements? High download limits or gaming pings or consistent high speeds (depending on what your line is capable of) or lowest possible cost (and possibly low quality)?
Put your phone number or post code into
here and post the results and we will be able to recommend a suitable ISP.