A lock in even exists with BT. If you are with BT so have an Openreach provided FTTP connection, you would expect to be able to swap to any of the other common ISP's, however when you try (as I did recently) you discover they don't support FTTP. The only exception is Zen which pitches its price only slightly below BT.
It was ever thus. Part of the challenge is the many faces of BT. So you may have Openreach FTTP, but the service would be via BT Retail or Business, who'd buy their connections from Openreach based on FTTP (GEA) per SIN 506. Then BT Wholesale takes that and turns it into FTTP (WBC) per SIN 509-
Which lists the many sins of BT. So for competing ISPs, they'd need to build their service from those BT products, and depending on whether they're an Openreach or Wholesale customer, have different processes and core/edge costs. If ISPs have already built out xDSL infrastructure for LLU, ie have their kit in BT's exchanges, and have space/power, then they might be able to use GEA. Otherwise it's usually WBC where Wholesale offers a bundled service to your PoP.
Which basically means it's tricky to compete on price or differentiation for basic FTTP. Especially as BT really really didn't want to offer a dark fibre LLU service, so control that by offering Ethernet. And it's not like the good'ol days of LES where you could just take BT's box off and end up with a fibre.
But such is politics and regulatory shenanigans. And also one of the problems with FTTP competition. So if it's a BT service, you might be able to migrate to another ISP that's using BT infrastructure. If the service is from a provider like Community Fibre, you generally can't because they may not have any wholesale service competing ISPs can buy.
That can sometimes be a problem with flats. So an alt-net does a deal with a developer for an MDU solution that can be punted to their tenants as FTTP. MDU's are simpler because you've got a captive market so can fibre up flats cheaply as it's inside plant, so civils costs are less. Then feed those fibres into a switch in a common area & fibre that back to your PoP. If residents are lucky, that might be via PON (Passive Optical Network) and a WDM filter pack muxing individual flats onto wavelengths, but that has distance limitations. So tenants may be stuck with the switch version, which means they're a VLAN on a contended backhaul connection. I've seen a few where tenants were sold 'up to 1Gbps' but the switch connection to the PoP as only 1Gbps.
But that's all part of the joy of designing retail networks. Article should also probably mention City Fibre as an alt-net as they've been active around the UK digging up pavements. And the digging part can also be a problem, especially around cities like London where councils get fed up with roadworks and can put an X year stop on construction. And then of course there's taxes.. So the rateable value of dark or lit fibre that has to be paid regardless of whether you're generating income from that fibre. Unless you're BT or Virgin, who got a sweetheart deal to pay less rates.