I don't know about the UK, but in Ireland very, very little of the Irish phone network or the power network was 'direct buried' (i.e. just stuffed into the concrete / soil with armoured cables). The vast majority of it is in ducts, or where it's not in ducts, overhead. So, laying fibre in most cases is just a matter of pushing micro ducts through and then blowing the fibre down those to the end users.
That's precisely what was done with the FTTC cabinet rollout, where the only major civil works involved in most cases were about getting 230V AC power to the cabinets from the nearest 'mini pillar' (ESB Networks connection point) and it's how they're extending that to full FTTH.
Eir actually set out some pretty strict requirements for how phone lines would access homes quite a long time ago. It's always been ducted, but it's evolved over time so that maybe about 20 years ago they added an ECU (External Connection Unit) which is like a mini version of an electricity meter cabinet and ducting going back to the nearest vault or telephone pole with inspection access at the bends. This was specifically done to future-proof for fibre. It's completely overkill for a POTS line but it's very useful for fibre.
Eir had a plan to either roll fibre, coax or just extra copper lines at some stage in the future, so they had a plan for this a long time ago, as they obviously knew this was around the corner eventually.
The state-owned power utility ESB Networks also has some very strict requirements for provision of ducting and inspection points which has allowed for very easy rollout in areas that aren't absolutely ancient.
ESB uses a very structured setup where you've a substation feeding 'mini-pillars', each of which feeds several homes. It's possible to quite easily push fibre through this and site splitters underground in the vaults.
SIRO is their joint venture with Vodafone for FTTH. That's running fibre over ESB infrastructure to get into homes via the ducts, overhead wires and ultimately coming out at the meter cabinet on the side of the house and into an ONT.
SIRO is also fully wholesale / open. So, it's got multiple ISPs using it. So far, just Vodafone and Digiweb but apparently Sky and several others may jump onboard too.
If infrastructure in the UK is mostly direct buried, it could explain why it's proving more difficult to upgrade.
--- How an Irish POTS line is installed --- :
---- How to connect an Irish electricity customer ---:
Full guidelines for a development:
OpenEir also gives you a huge amount of information about their network and how it all works, what's available where, what they're planning and so on :
(Geeky I know, but this is a geeky site and I have no idea how this compares with the UK or elsewhere)