Unfortunately it’s a bit of a postcode lottery, and it mostly depends on the infrastructure in your area.
Some providers can only supply households on their own cable networks, such as Virgin, Gigaclear and KCOM. Most others are reliant on the Openreach network which covers most of the country – but not everywhere. Often rural areas get the short straw because the expense of running cables can be prohibitive.
To get fibre deals, you need to be in an area where the exchange and cabinets have been upgraded to fibre optic cables and there needs to be spare capacity available for your connection.
Providers may choose not to supply your postcode if they aren’t confident of giving you a good speed.