Most cost and labour effective evergreens for windowsill planter?

Bossypants

This is my first foray into this forum and gardening in general, so apologies in advance for any stupid questions.

I've just moved to a new place and would like to put planters on some of the window sills. The wall in question is north-facing and somewhat exposed, located in SE England. I am hoping to find a hardy evergreen shrub-type plant which grows upwards (rather than hanging over the edge), but when I went to the garden centre they just pointed me towards the general Evergreen section, which was not all that helpful.

If it could be something that flowers, that would be a bonus but not essential.

Thanks in advance for any suggestions!

Edit: Would heather work?

Fuzion

Box balls look nice, you could then change the bedding plants between them as here with white geraniums. (But the geraniums and lavender in the picture don't like north facing particularly - there are other options though).