You need some kind of connection. Phone/copper wire is usual but cable is an option. Point is that the Internet access supplier need not be the same firm as who provides the physical connection. Directly or indirectly BT probably supplies your connection.
Given that you need one, it's best to accept the cost would be much the same however they chose to split it between voice or data. If they made calls free the data connection cost would rise, and if they made data free line rental for the calls would rise.
Aside from going to comparison sites to see what is available in your area, I'm unsure what to suggest. You can only go for the best deal at the time of looking.