You'll need to avoid anything tall as you've discovered.. Have you thought about alpines? Not as showy for a window box but at least they should survive and there are plenty to choose from - most garden centres will have a good selection.

How much sun do the boxes get and how often do you water and feed? Lavenders and Hebe tend to be tolerant of wind but not wind and no water and no sun. Can you put up some glass or fence around the box to give some sort of wind break

Thanks all...a windbreak might be the answer!