As the "outsider" which I am, I would appreciate an "education" concerning "BT openreach".
A long time ago, British Telecom owned and operated 99% of the telephone network in the UK, including the exchanges, the copper wires to individual premises and all of the rest that connects them together. If you wanted a telephone, you got it from BT. There was no other choice, and you paid whatever BT told you to pay. If BT didn't think you were worthy of renting a telephone, you got nothing.
Then it was decided that competition would be a really good thing, and other companies could provide telephone and internet services using the wires that BT owned.
After much wrangling and excessive expenditure, Openreach was created as a wholly owned subsidiary of BT, which owns and maintains the manky old copper wires between the exchange and the individual premises.
Today you can obtain service from many companies, some of which just resell services from others, some which actually install their own equipment in the exchanges.
However in every case, it's still the same old copper pair from the exchange to the premises, and if those go wrong, it's Openreach that has to repair them.
For the end user when a problem occurs it's:
1 - contact the provider that you buy service from
2 - provider fobs people off with useless solutions such as unplugging, switching on and off, etc.
3 - provider eventually sends request to Openreach to fix the problem
4 - Openreach arrive and do whatever they do
5 - Fault may be fixed. If not, go to 1 and repeat.
End users can't contact Openreach directly.
Openreach owns the cabling up to the master socket, and the master socket itself. Anything after that such as extension cabling and extension sockets is the property owner's responsibility.
In theory only Openreach is supposed to alter the master socket and any wiring before it. Reality is often different.
In terms of internet service, most of the country is still on those manky old copper wires. Most areas have what's wrongly sold as 'fibre' which is really just fibre to a cabinet in the street with manky old copper wires to the premises. Theoretical speeds 80Mbit download, 20 upload. In reality usually a lot less, particularly if the cabinet is far away, which they often are. Or the copper wires take a very long route, have multiple joins, corrosion, are run adjacent to 100s of other pairs or are that flat twin non-twisted pair cable that was used extensively in the 1970s.
A non-trivial number of locations still have manky old wires direct to the exchange, and for those the absolute best is a download of about 16, but in reality will next to useless as many of those are very long lines and barely work at all.
Some more fortunate people can have fibre into the premises, but that's still the minority. Currently being installed to more places, but it's a long, drawn out process that should have been completed a decade or more ago. Will be another decade or more before majority coverage is reached.
Due to the above, the service you get depends entirely on the state of the Openreach network in your location. You can buy service from a wide selection of providers, but the limiting factor is the network. If you live somewhere with a rubbish network you get rubbish service - but will pay the same as those in more fortunate areas.
The only other option is locations where companies have installed their own independent network cabling such as Virgin Media and others - but that's mainly a selection of some parts of larger towns and cities.