In reply to Cú Chullain:
I use Andrews & Arnold (https://www.aa.net.uk/). You will find cheaper but I'm pretty confident you won't find anyone any better for customer service. I use them at home, work & for my parents. They will sell you a bare copper pair, without an associated phone line.
They are an independent ISP and use wholesale lines provided by BT & Talktalk. They have a constant 1 ping per second monitoring of all the lines they provide. You have full access to all the data monitoring for your lines and are free to alter anything you want. They aggressively hassle BT & Talktalk on your behalf whenever there is a problem. They also have a strong privacy & internet neutrality policy, and have argued the case before select committees. They have not implemented any of the torrent/porn filters imposed by the larger ISPs. A firm run by nerds not shareholders.
They have an impressive 'we will fix your line' policy. If you have a problem line with another provider, you can transfer it to them and they guarantee to take on the fault and sort it out. They will force BT to re-cable your house and swap exchange hardware if necessary.
The best thing though is that they have no call centre. You call them and a network engineer answers the phone. They have diagnosed and fixed awkward firewall rules for me, remotely, in seconds. I had an outage on my parents broadband line a few months ago. I could check online using their diagnostics tools to see how long the line had been down, I could reset their router remotely. I couldn't find the problem so I called them. Engineer answered immediately and within 2 minutes he had extracted the cause of the fault from the BT Openreach system. It was damage to some of their hardware in the street. He had direct access to their logs and could tell me the anticipated time for the job to be fixed.