Pre-Brexit, it was the case that if you left the UK with your mobile phone and used it in the EU then, as far as your phone bill was concerned, it was as if you hadn't left the UK. Calls, text and data were all exactly the same as if you'd stayed at home and made the same use of them.
Now, that isn't the case. You can be stiffed for much more money.
Some providers have not increased prices though, yet at least, notably O2. Perhaps because they're operated by Spain-based Telefónica.