ISPs that build and maintain their own infrastructure, such as Virgin Media and KCOM, can only cover postcodes that their cables can reach.
If they have no physical way to connect to you, they cannot provide you with an internet service.
Most other ISPs use the same infrastructure – the BT Openreach network.
This is a very old network and it is made up of copper telephone lines and fibre optic cables. These telephone lines cover almost everywhere in the UK but the coverage of fibre optic broadband cables is somewhat limited.
ISPs have to rent space on the Openreach network before they can sell a service to customers. Some ISPs rent space that covers the whole country, while others may choose to only cover specific areas.
This is why different ISPs and broadband speeds are available in different locations.