Judging from the Wikipedia list of UK ISPs[1], providers use a variety of different technologies, have different coverage areas, lease lines to smaller ISPs, provide different bundles with other services that they provide, and so on. Maybe one service is known to optimize for throughput, while another offers slower speeds, but also lower latency for gaming, or something. There are a lot of services besides just the internet connection that ISPs could provide to their customers (or not provide, and market themselves as the more economical option).
[1] https://en.wikipedia.org/wiki/List_of_broadband_providers_in...