Why can I not use mobile data whilst in Jersey

I have plenty of uk data left on my home plan yet cannot use mobile data after arriving in Jersey?

I have just come back from Jersey and I also found I could not get a signal to use mobile data. I’m very disappointed

The annoying thing is that I paid for a roaming plan as I know that Jersey wasn’t covered

Jersey is not part of Uk providers treat as foreign country outside eu. Most providers treat that way and not covered by eu roaming.

Jersey is not actually part of the UK
While Jersey is a British Crown Dependency, it is historically and legally a self-governing island. It is not part of the United Kingdom, nor has it ever been a member of the European Union.
Because of this independent status, UK regulations and standard UK mobile coverage maps do not legally apply there.
2. Independent Mobile Networks
The UK's big networks (EE, Vodafone, Three, and O2) do not own or operate the mobile masts on the island. Instead, Jersey has its own independent telecom providers (like JT and Sure).
When you use your phone in Jersey, your UK provider has to pay these local Jersey networks a "wholesale roaming fee" to route your data, calls, and texts. Because the UK providers are being charged for your usage, most of them pass that cost directly onto you rather than absorbing it.
3. Left Out of "Roam Like at Home" Laws
When the EU introduced its "Roam Like at Home" regulations in 2017 (which banned roaming fees across Europe), Jersey was excluded because it wasn't an EU member.
Even though the UK has since left the EU and many UK networks have brought back European roaming fees, Jersey remains categorized under its own separate, pricier roaming zone for many providers.
A Quick Network Check:
Not all UK providers charge extra. A few networks (like O2 and Asda Mobile) sometimes include the Channel Islands in their domestic zones, while others charge a daily pass, and some charge hefty pay-as-you-go rates. It is always worth checking your specific network's roaming page before you go!
If you are staying for more than a couple of days, getting a temporary eSIM or a local SIM card from a provider like JT or Sure is usually the cheapest way to stay connected.