From ofcom
Customers using the EE, Vodafone and O2 networks with an older device will still be able to make calls and send text messages after 3G switch-off. These services can use the 2G network, which isn’t being switched off yet. But the 2G network was not designed to work for accessing data services, so customers with older devices will no longer be able to access most of their mobile data services after 3G switch-off (although where older devices can connect to Wi-Fi, customers can still use this to access data services).
Three doesn’t have a 2G network, so if you’re using the Three network and have an older device that doesn’t allow you to use 4G or 5G, you’ll need to replace it to make sure you can continue to make calls, send text messages and access mobile data after 2024.