Since roaming charges have been abolished within the EU (and the UK is still within the EU), you'll pay exactly the same when making calls in other EU countries as you do when making calls in the UK. (That applies to
all phone service providers).
So if you use 1p Mobile, which charges 1p per minutes for calls in the UK, that's all you'll pay when elsewhere within the EU. Similarly 1p Mobile charges 1p per Mb for data in the UK, so that's what you'll pay elsewhere in the EU (as well as in EEA countries that aren't within the EU).
https://www.1pmobile.com/international-and-roaming.taf
If you intend to use your phone outside of the EU/EEA, you'll need to look at the various bundles on offer from different phone providers. (It can often be much cheaper to take a phone which isn't tied to a UK network and then to buy a SIM card locally).