An adult travelling with a minor who does not have the same last name will twig suspicion. This may sound like overkill, but having gone through hell multiple times trying to get a younger sister on flights (we have the same last name), I would have a signed, possibly notarized letter, from both her parents giving you expressed permission to take her on the trip. Most of the problems I've faced have been with the airlines themselves, and not the countries upon entry, but I did have an awful 6 hour ordeal at the US embassy in London in December. Granted if you sign your own forms and are on seperate PNR's, you'll have no issues at all. Notarizing the letter would require an appointment in Grosvenor Square, which is an ordeal in itself.