What is OFNL?
OFNL (Open Fibre Networks Limited) is a UK operator of fibre optic networks - similar to Openreach, but considerably smaller. Like with Openreach, you buy your internet connection through resellers, which you have likely never heard of.
OFNL operates primarily in newly built residential and commercial developments, generally exclusively - in other words, you don't have a choice of whether or not you want to use OFNL.
OFNL vs. other internet options
Most households in the UK have internet through an Openreach reseller, like BT, Sky or Talk Talk. Some households have fibre options from companies like Virgin Media, Hyperoptics and other companies. These are generally available in addition to Openreach-based providers.
OFNL, like many alternative networks, operates a FTTH (“fibre to the home”) network, also known as FTTP (“fibre to the property”): fibre optic cables that go directly into your house or flat. In theory, this offers the fastest internet connections, but OFNL also offers several broadband packages that aren't any faster than copper-based internet.
Is OFNL any good?
OFNL offers packages with faster top speeds than non-fibre providers. In most of their locations, like most other UK fibre providers, OFNL tops out at 360 Mbps for residential connections. This is better than what copper cable connections offer, but does not compare well internationally. Gigabit connections are only available to businesses.
It is difficult to get good data on network reliability. Personal experience suggests that reliability is similar to Openreach: The author has seem very few drops in connection, and they've been gone after a minute or two.
Anecdotal stories seen on review sites suggest that bigger outages might not get addressed during weekends.
Do I have to use OFNL?
In short: Yes.
At sometimes, OFNL add exclusivity clauses to their contracts with developers, which can even be found in individual home owners' leases. Even after that period of exclusivity expires, other fibre providers are unlikely to invest in connecting to your development, and might not even get permission by the development's freeholder to do so.
Can I move my broadband contract to OFNL?
In short: No.
OFNL resellers are different than those for Openreach, so there's no way to move your contract when you move. If moving to an OFNL-served property would result in an early contract cancellation fee, call your current internet provider and tell them about the situation - they might be willing to let you part ways for free.
A note on Sky: You might have come across “seethelight”, a company owned by Sky. Unfortunately, this does not mean you can simply move your contract over - they're completely separate organisations.