Mobile phone companies make a lot of money from roaming charges and it's that money that helps them give away £400 phones for free when you take out a contract. Nobody likes paying extra charges but it's a fair compromise - the small minority that use their phones abroad on a regular basis pay the extra cost of using it rather than the majority of mobile phone users subsidising their usage.
In Europe it's as common for people to go to a neighbouring EU country as it is for English people to go to Wales or Scotland but they are still in the minority. It's not as common in the UK and Ireland though, thanks in part to the EU's environmental laws that make it prohibitively expensive to travel by air but mostly because we're an island and it's not convenient.
The mobile phone companies won't absorb the extra costs, they will pass them on to all their customers. We will all pay higher bills to subsidise the mobile phone usage of a minority of customers and in the UK and Ireland we will benefit the least. The EU is not a country, the countries in it are not states in a country like the USA despite the EU's insistence on referring to them as "states" to try and get people to think about them in the same way. It is not unreasonable to expect to pay a premium for using your mobile phone in a different country.