The answer depends when you last re-contracted or took out your current plan - whether this was pre or post-7 July 2021.
If you took your plan out before this date, then you can use your UK allowances in the EU at no additional costs. If you took your plan out since 7 July 2021, then charges will apply. If you're going to use your phone a lot in the EU, then the £10/month "Roam Abroad" pass is well worth considering.
You can check roaming costs for Greece, including selections for your specific circumstances, using the online tool at https://ee.co.uk/help/help-new/roaming-costs/countries/greece
