does anyone know if you can use mobile broadband (ie the ones you pay a monthly subscription and it's a little device that you plug into the USB port on your computer) outside the UK? We're going to France and I want to know I can get the internet in the place we are staying.

Depends on coverage.
Provider.
Locked? device.
Plan [roaming fees]

It's possible. But without more info nobody can figure out if your device will work.

Wifi is fairly common in France, especially in cities. Have you researched whether it's available in places you are planning to go?

- Roz