There is a network that was built using taxpayer money (Openreach) and they’re required to lease it out to other ISPs. However, other companies are free to build their own and only use parts of the Openreach network. Virgin Media (formerly NTL) have built their own network but most other companies only build own small sections here and there and plug into Openreach for the rest, but Virgin don’t lease their network out. So you’re basically using Openreach or Virgin Media.
There is a network that was built using taxpayer money (Openreach) and they’re required to lease it out to other ISPs. However, other companies are free to build their own and only use parts of the Openreach network. Virgin Media (formerly NTL) have built their own network but most other companies only build own small sections here and there and plug into Openreach for the rest, but Virgin don’t lease their network out. So you’re basically using Openreach or Virgin Media.