Rules which kept the cost of calls, messages and internet usage from your mobile phone the same in the EU as it was in the UK, no longer apply to UK mobile operators.
As such, the availability and pricing of mobile roaming in the EU is a decision for UK operators. You should contact your mobile company to find out about their roaming fees.
For more details on mobile roaming: https://www.gov.uk/guidance/using-your-mobile-in-eu-and-eea-countries