Regarding juggling sim cards: I recently traveled between a few EU countries using a 3 UK pay-as-you-go card (with the their roaming package http://www.three.co.uk/Discover/Phones/Feel_At_Home). It worked well, and even in the middle of nowhere in Italy, I had good coverage. It was a PITA to top-up before I left from the US. But if you are already in the UK, that is probably a non-issue. I think it was 15EUR for unlimited data + N/minutes & texts.
I am sure there are probably downsides, but it might be something to look into while Fi is still in infancy.
Regarding juggling sim cards: I recently traveled between a few EU countries using a 3 UK pay-as-you-go card (with the their roaming package http://www.three.co.uk/Discover/Phones/Feel_At_Home). It worked well, and even in the middle of nowhere in Italy, I had good coverage. It was a PITA to top-up before I left from the US. But if you are already in the UK, that is probably a non-issue. I think it was 15EUR for unlimited data + N/minutes & texts.
I am sure there are probably downsides, but it might be something to look into while Fi is still in infancy.