Here you have three choices:

1. Go with BT.

2. Go with a cable company. Ideal if you want TV as well. Lower installation charges (90% lower). Completely separate network between you and the exchange (so can be good if your BT line is too noisy for a modem and they won't fix it). But poorer quality of service and non-existent customer service.

3. Go with BT for the line rental, but pay another service provider for your calls.

There is no distinction between local and national providers. Although as in point 3, you can choose to use another provider for certain calls if you wish.