Hi Cecile, ages ago I wondered about collard greens but have never found them in the UK.
They are very similar, however, to what we in the UK call 'spring greens' and according to good old Wikipedia are classified in the same cultivar group and are genetically similar, see http://en.wikipedia.org/wiki/Collard_greens
The pictures look the same too, so I decided to eat spring greens and think of them as the same as collards
Don't know if I was right to do that, but as there's no reference to 'spring greens' in the food typebase here, it seems the best we can do. Unless someone more knowledgeable than me knows better, of course!
You can get them in Tesco's, near broccoli and kale, etc.