I admire your sense of adventure! Are you going to to be travelling alone or with a companion (other than your dog )? I think that the logistics of driving down to south of France or across to Italy for the ferry (along with the hassle of finding accommodation on route that will accept a dog staying) would put me off. Sorting out the travel insurance might turn out to be the easiest aspect of this trip to plan for. Apart from the ferry from Sardinia all of the ferry crossings seem to be in the region of 24 hrs long and even the ferries from Scility seem to be 10-12 hrs long. Will they allow your dog to stay with you or will it have to be treated a 'freight'?

http://www.directferries.co.uk/tunisia.htm

Also it occurs to me that it might be difficult to find a vet in Tunisia who deals with small animals/household pets - they have never struck me as being a country that tends to keep dogs as pets and there is a cultural aversion to keeping dogs as pets in most Muslim countries. I appreciate that this is a very broad generalisation but are you sure that the owner of the villa will let you keep a dog in it with you? And are you absolutely sure that it will be possible to find a vet who could provide the appropriate paperwork for re-entry into the UK? It would be heartbreaking to go to all that trouble to find that you couldn't comply with the pet passport requirements on your return and had to put your pet into quarantine kennels after all.

If you haven't found it already the following website has lots of useful information and might help you find answers to some of these questions.

http://www.pettravel.com/immigration/UnitedKingdom.cfm

SM