The additional allowance of mobile data can be used in the UK or when roaming in any of the 40+ countries in our Euro Zone. View the full list, here: https://guides.plan.com/pro-europe/
To add a Data Boost¹ and provide additional, reduced cost, temporary UK/EU data, go to:
PRODUCTS > DATA > DATA BOOSTS > BOLT ON/DATA BOOSTS > [select user] > ADD
¹account permissions may apply. Contact your account administrator.