Tortoises do not fall under the EU pet passport scheme, however travelling with a reptile is relatively simple within the EU.
You may come to France with a reptile as long as you are not travelling with more than five and you are their owner or responsible for them on behalf of the owner, and that you do not intend to sell them on in France. You need to report to the customs service (douanes) on entry to France and make a declaration regarding this.
You also need to take with you a health certificate issued by a vet from one to five days before entry of the animal into France. This should show the tortoise is in generally good health and not showing symptoms of certain illnesses typical to their species.
An Agriculture Ministry expert from the animal transport office said it is “preferable” if the certificate is in French but not obligatory. A vet from one large vet practice, the London Vet Clinic, said: “We do not translate health certificates in to the language of the destination country and as far as I am aware this is not a necessity.”
The UK government states that there are no requirements for bringing a reptile (back) into the UK from an EU country.
You also need to check with your chosen travel company, for example with British Airways pets have to travel in the hold in a suitable transport case, whereas Easyjet does not allow pets on board. Eurostar only allows guidedogs, whereas there is no restriction or cost involved in taking a reptile in your car via Le Shuttle (Eurotunnel).
If the tortoise is on the endangered species list (see cites.org), you need to take a CITES cert showing that it was legally acquired.