What it actually means
Most UK broadband and mobile plans advertised as 'unlimited' sit on top of a fair use policy. The headline says you can call, browse or roam without a hard cap, and for the typical household or small business that's true in practice. Behind the scenes the contract reserves the right for the provider to act if a single user's behaviour is so far from normal that it implies commercial resale, a fault, or abuse.
In practice FUP shows up in three places. On unlimited UK call plans it prevents someone from running a call centre off a residential bundle. On mobile data it protects the network from a tiny number of users consuming many terabytes a month. On EU roaming it stops UK SIMs being permanently used abroad, since UK price controls aren't designed to subsidise long term overseas living. When a provider does act, they should warn you first, explain what's triggering it, and give you a route to either change your behaviour or move to a plan that actually fits.
At home
What this looks like in the house
If you've ever wondered whether streaming all evening, gaming with the kids and running video calls counts as 'too much', it doesn't. Fair use isn't aimed at heavy household users. It's aimed at the very small number of cases where one line is being used like a business or where a SIM has effectively moved abroad. For the vast majority of homes the word 'unlimited' on your bill means what it says in lived experience.
In business
What this looks like at work
For a business the FUP matters most around mobile roaming and around unlimited UK call bundles. If your team genuinely spends months at a time working from the EU, a consumer SIM with a roaming FUP isn't the right tool. Equally, if you're running outbound sales from a single line that's badged as 'unlimited home calls', expect the provider to ask questions. The honest fix is a plan priced for the actual use, with the call profile or roaming pattern written into the contract.