If you have a 4G device and a 4G UK contract, you’ll be able to use 4G abroad, wherever it’s available. Using 4G incurs no additional cost to your standard roaming charges.
If 4G isn’t available, your device will switch to 3G or 2G.
Can’t get 4G abroad?
It may be that you don’t have a 4G-ready device or bundle, or that there’s no 4G in the country you’re visiting.