Mobile internet..? Options? French Network or UK?

Hi, wonder if anyone can advise.

We need to work in France in a fortnight and we won't have wifi installed yet; I need to work from my laptop and need a dongle - or even better; one of the cellular boxes that creates, in effect, a wifi signal that anyone can connect to, like this:
http://www.gloopy.co.uk/user-reviews/ee-osprey-pay...

Best to buy in France? If so, what's the best option for this? Any advice apprciated. I note that 'Three - here in the UK - do a dongle that you can use in France at no extra cost. It would be handy to buy in the UK but I'm not sure about Three as a network. We alreadyd have an EE box (as linked to above) that I need to check whether we can use.

Thanks for any tips.

Would roaming not potentially be better in as much as the UK dongle would pick up whatever was the best signal / network out there? Maybe that's not how it works though...