I have just bought a pair of Canon 18x50 IS binoculars mainly for overflights, and they are superb. You can clearly see the airliners' colours when the magic button is switched on. Three downsides: they are heavy, the batteries last no time at all, and they are pretty expensive - over 600 quid!
Not sure whether I am allowed to do this on the forum, but I would highly recommend Ace Optics in Bath - very fast, friendly service and their website is helpful too...Just google them and they come up near the top of the page.