I think I would go for Berberis!

It is prickly and it looks quite nice all the year round, with its coloured leaves and flowers.

There are some pictures and cultural instructions here;

<url>http://www.thegardenersalmanac.co.uk/Data/Berberis/Berberis.htm</url>

I agree and maybe some Holly, like a Hedgehog Holly?

In 12 inch pots, the only berberis that will grow well and cope with being so contained is Berberis thunbergii atropurpureum 'nana', or B. 'Admiration' or B. 'Bagatelle'. Most of the other Berberis are large shrubs which will not achieve their full potential in pots only 12 inches deep. Note that the 3 varieties mentioned are all deciduous. Is there no soil in the area to plant in?

oops!

I didn't see the word " pots" I assumed around 12" high.

Which now means I take back what I wrote and agree with Bamboo.

Is it not possible to plant them into the ground? this will give you more options!

you mite not be allowed fencing but there may be a loop hole wear you can use trellis ? you cant have fence much above 6` round here but you can have trellis as high as you like . its worth a thaught .