@EvieLynch Hi E & welcome to the 3Community. Not to worry while you're in the UK your EU roaming allowance will work. With your plan, you'll get an allowance of 28GB for data usage, & you'll be able to make calls & send texts using your Flexi Unit allowance.
Once the allowance is used up you'll be charged at the roaming rate. You can check on the roaming rates here. The best way to keep an eye on your allowances is to check your My3 account either via the app or website.
If you have any other questions let me know 😊