Not sure if this has been posted elsewhere yet, but some changes in European regulations (capped tariffs) are coming into play this summer. Here is a link to a news story and snippets. Apparently you will be able to pre-select which network and plan you want to use when roaming.
BBC News - EU sanctions cheaper mobile-phone calls within Europe
http://www.bbc.com/news/technology-17539421
http://www.bbc.co.uk/newsbeat/17188133
From July, using mobile data in Europe will not cost more than 70 cents (56p) per megabyte - far less than current rates.
The first changes will come into effect from 1 July. Calls will be capped at 29c (23p) per minute, plus VAT.
The EU said that from 2014 customers would be able to choose their mobile networks upon arrival in a country, or signing up to a contract before leaving.
Currently, mobile users are forced to use their standard domestic operator when travelling abroad - or to use alternative arrangements, such as a cheap pre-paid SIM card.
Under the new regulations, customers can choose a different operator with a more attractive travel tariff before leaving - without changing their number.
By July 2014 customers will also be given the option to shop around for the best deal and sign up for a separate mobile contract using their existing number when going abroad.
They will also have the option to directly select a local mobile network in the country they are visiting.
The new roaming charges will progressively go down and by July 2014 the aim is that roaming consumers will pay no more than 19 cents (15p) to make a call and 20 cents (16p) per megabyte of data. The cost of receiving a call will fall to 5 cents (4p) and sending a text to 6 cents (5p).