Theres a lot of history here - most of which comes from National Grid Transco, which was highlighted earlier.
From what I can piece together - Transco back in 01 was owned by Lattice Group who also had a subdivision called '186k' (indeed you can still see this on some manhole covers). 186k deployed fibre alongside Transco gas pipelines.
(If you map the National Grid gas backbone and Zayos network, they match pretty much 1-4-1 on the routes taken)
Fast forward through telecom boom and bust -
- 186k signed up Three as a backbone fibre provider
- Lattice also had a second company called SST providing 'telecom tower services' (probably also used by Three)
- Transco went to merge with National Grid, but it appears they had to divest itself of 186k
- A company called 'Geo Networks' brought 186k (Geo being Hutchinson, aka Three)
- Geo Networks eventually struggled and was brought out by Zayo
TL;DR -
- 186k laid the initial fibre
- Three were a customer of 186k
- Geo (Three) brought out 186k
- Zayo brought out Geo (Three)
Are they using it for tower backhaul, probably, but only someone from Three/Zayo could answer the exact function(s) of the building(s)