The answer significantly depends what plan you're on, specifically whether any of the Roam Further variants.
You can view roaming costs on https://ee.co.uk/help/mobile/roaming/roaming-costs - if you don't, then you're looking at £25/week as per https://ee.co.uk/roamingrefresh
Regardless of your billing add-ons, remember to ensure the free roaming facility is enabled on your SIM which allows access to foreign networks - without it, you won't get coverage outside the UK. Text ROAMING to 150 before leaving the UK.
