We have some relatives visiting from the US who have unlocked phones. We haven't done this for a few years, but I think now that the EU allows free data roaming, it should be easy to get a SIM to use in both the UK and France for a few weeks? Or are there going to be restrictions on roaming on a new payg SIM?

Also, assuming this is the case, does anyone know which network will give the best data speeds in France? I personally use Three but have been underwhelmed by their roaming speeds in Paris, so I don't know if anything else will be better.