No-water, flowering hardy shrubs/perenniels for 6 month shade -alkalai soil?

I have a friend who lives in London (where there is substantial frost), and has a garden space against a north wall, such that it gets sun only in the summer, who would like plants (could be annuals, perenniels, or shrubs) that flower even in winter, and never need watering besides the natural rain (the wall provides a bit of a rain shield as well).   It can't be a tree, because she is afraid of roots endangering the neighbor's foundation (the wall).  She does not like Mahonia.  I was tempted to suggest an artificial plant, but did not.  She would also like to amend the soil (which is friable rather than clay - a previous owner had apparently removed most of the natural clay) so that it is slightly acidic (ground water is apprently naturally basic there).  She has firm convictions that the use of any peatmoss to change the soil pH would be an ecological crime, based on its non-renewable character in England.  Any suggestions as to soil amendments available in England or plants? 
 
Richard Starkeson
Sanb Francisco
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