More specifically
Openreach sells products only to ISPs, it won't deal with consumers at all. The main products it sells to ISPs are:
* ADSL last mile to a customer (in most places ASDL2+ but in some places it's plain ADSL because they never upgraded) at the maximum speed the line will do
* VDSL capped at 40Mbps down / 10Mbps up over last mile to a customer [in most but not all places, unless line is too long]
* VDSL capped at 80Mbps down / 20Mbps up [ditto]
* FTTP at 330, 500 or 1000 Mbps to the customer's premises. [ Only if there is actually a fibre, most won't have one]
* Line rental to make a physical line work with any of the above.
* Backhaul from the customer's exchange to your preferred POP where you take over the Internet access
A small ISP buys the line rental, backhaul, and one of the actual last mile packages, bundles that up with maybe some webmail and telephone support, and sells that as an Internet service for a nice margin. They need a POP somewhere vaguely sensible, with enough bandwidth to service their customers, and that's them done.
Bigger ISPs might arrange to have their own fibre at some exchanges, they don't buy backhaul from those exchanges, they pick up customer traffic themselves, but they buy everything else.
One interesting thing that you can see as a result of Openreach was that the true "line rental" cost Openreach were charging to make the line work was falling slowly over the years. But the _advertised_ line rental price had been rising enormously, because ISPs hid it in their small print. They'd say "Superfast Broadband only £5" and then in tiny print "(plus £20 pcm line rental, offer expires after 12 months, normal price £28 per month)"