"Surely for the bigger providers (i.e. vodafone) this should be a non-issue as they already have networks in most countries so they are only paying themselves..."
That would be one problem with this plan: it screws smaller operators who don't have a heft pan-European footprint.
Some comments point out the *cost* of a call now is trivial. In a way, that's right - the problem is, the *price* of that call - even for one telco charging another - is not.
Supposing I had a UK mobile network. I charge a flat 2p/min for calls. (As, in fact, my current provider does.) If you roam to France, Orange charge me 10p/min; go to some small island, maybe I get charged £1/min because there's only one operator there. Now the EU demands that all of those be the same price to customers - do I charge everyone £1/min so I'm not getting ripped off any time someone visits the island? 3p/min to everyone, so UK customers have to subsidise tourists and make that island's monopoly rich?
That's the problem with the "one price" proposal: there is no single fair price to customers, when the goods in question differ in wholesale price widely! If they were to regular wholesale roaming charges, then cap the retail roaming price based on that, it would be fairer all round. Yes, calls in Croatia would cost me more than they do at home - that's because the service in question actually costs more to provide! (Of course, Three have managed to iron out the difference, for *some* countries; hopefully that will continue and spread - but it certainly isn't universal, and perhaps never will be.)