I think if a company wanted to compete with BT Openreach they can it is just impractical in most scenarios. There is also a cable company with its own infrastructure covering about half the country but they aren't interested in increasing coverage.
Because the local loop is regulated you can then have competition at the level above that with different pricing and service offers, so you can choose BT or at very similar price a number of others or for a little more really good ones offering IPv6 and good support (within the limits of the Openreach managed lines).
Is it perfect - definitely not but it is a hell of a lot better than the situation in much of the states and I believe than pure unregulated market in this sector.