That's GC's own fibre network; there are also many more of these in the UK, some covering more ground, some covering less.
Generally, if you order from GC, you'll get a tail from BT everywhere other than major cities (in my experience).
You could order a service from them - generally they provide point to point 'leased lines' but I'm sure they have an internet presence you could connect to - if you have a lot of spare cash.
There is no GC street level network in the vast majority of the country.
Comms is hard