I travel alot, but it differs with every country's custom regulation. In any case you would need a healh certificate authorized by an aprooved veterinarian. Depending on the country, they can hold you animal for quarintine... which can be months, but I'm sure you know about that. If your going from hear to the U.K i know it isn't that big of a deal as the reptile trade between England and U.S is thriving. Just make sure you check the regulations for the specfic country.
In the repiles case. A 10 to 20 hour flight can be quite brutal possibly fatal. Assuming all goes well he/she will hopfully recover in a week or so.