Re: My money's on Vodafone being the first to start charging
I suggest you take a close look at your existing contract.
EE for example already includes EU roaming calls (ie. when you take your phone abroad and make calls) in your call allowance, however calls to the EU27 (eg. UK to Ireland) are charged at "the standard rate" ie. they are treated as international calls.
Obviously, you have the option of buying an international call add-on that gives you a call allowance for selected countries.
Personally, I expect many operators will in the first instance amend their fair use policy, before offering contracts without EU roaming as standard.