We have had some dealings with Orange whilst moving from one house to the other and every single thing we have been told as fact is rubbish.
If you use a 3G cle (dongle) attached to your orange.fr computer in the UK it will incur no extra charges. (It incurs vast roaming charges.)
If you wish to cancel your telephone/internet on your old line write to an address in Lille. (You need to write one letter to an address in Lille to cancel your internet and a separate letter to the address on your bill to cancel your telephone line. It was only Elder Son ringing the old number and getting my answer machine message that alerted us to the fact that they haven't cancelled it.)
You cannot cancel your new account for 12 months. (Yes you can, if you are moving to a place that Orange doesn't cover.)
Oh and if you do send a letter, make sure it is recorded delivery or they won't acknowledge that it exists.