In the UK, they split the infra provider (Openreach) from the consumer company (BT). So it's no longer BT giving access to the other providers.

In theory, BT has no special access to the infra at all, and they're on a level playing field with other providers.

That may not be perfectly true in practice, but my impression is there are no large differences between providers on the same infra. Choosing between providers mostly comes down to packaging and customer service in the end.