Everyone will have their own favourite, perhaps rather tender shrubs but it would also depend on how far into London and how cold the winter would still be. If frost wouldn't be a problem then the one shrub I would want to grow would be Loropetalum chinense var. rubrum 'Fire Dance', Chinese witch hazel.

Aspect's quite important! Also the size and any requirement for privacy and if you're going to be using it for anything (like sitting out, eating etc) any chance of a photo of the area? Is there access to water up there?

Their are so many plants you can use, so long as you place them in the correct aspect and look after them properly then you can make yourself a lovely peacefull relaxing roof top garden, to do it properly you need to make a good jumble of various sized pots, bigger ones to the back, don't be afraid to experiment, use some of the architectural plants, use perennials and put in some bedding plants, geraniums, petunias etc for dramatic affect Bamboos, but not in the waist high areas, these will give you so much joy the gentle rustling sound in the wind whilst sipping wine on a summers evening will transport you to a place of peace away from Londons hustle nd bustle, but secure well from high wind, virtually all the shrubs you can buy from the garden centre I have planted in pots over the years with fantastic affect, but you have to look after them, watering and feeding and re potting from time to time, but well worth the effort to escape from that crazy hectic city life, done properly you will have something like paradise to look forward to when your days work has finished.

Thanks for your comments, I shall pass them on. That sounds very romantic Julien. They are novice gardeners but quite excited at the prospect of creating a beautiful outdoor space.