I agree that it 'depends'. I disagree that Flying Blue would be the best option. Flying Blue now has a short expiration date similar to that of the American carriers. However, it has the added problem that the only activity which will count towards maintaining your account is flying. You cannot use hotel points, etc. to extend the validity of your miles. (In some countries, a FB credit card will extend them, but not all - I am not sure about the UK).