escallonia form a substantial shrub and I think would struggle in troughs. how wide/deep are they? try the hardier fucshias Thalia is a bit tender for me even in a sheltered spot.
some of the dwarf Euonymus might fit the bill as would a dwarf holly.

I was going to say Euonymus fortunii too but I suggest it so often I was a bit shy of saying it again...I have five of them, three in a dry bed on the north side of the house and they are bright all year round. They would be fine in a big enough trough. They are tough but aren't fast growers though.

Euonymus are great, but I am wondering if there is room for a climbing Hydrangea. Only if the troughs allow the roots though. Otherwise, why not evergreen ferns, Bergenia or Tellima. They won't be tall as shrubs, but will be pretty. Another great climber for shade is Fatshedera lizei. I have two in large planters (18" square ) and they are doing brilliantly.

Berberidopsis corralina does well in shade too, and I have also grown the chocolate vine - Akebia Quinata in a shady spot very successfully, but you won't get so many flowers.

I would go for Heucheras. Preferably the purple or darker shades. They will not take up much space and because they are evergreen, will give all year round interest. The only other consideration is our old friends, the vine weevil are quite partial to them but easily dealt with in a container.

Good idea Jimmy, although in a shady space I think I would go for the lime green ones to brighten it up a bit. Its harder to see the dark ones in a gloomy spot. :)

You can miss Heuchera 'Fire Chief' though.

No! Thats very true!

Thank you all so much for your advice. I think I shall give Escallonias a miss and look for some Euronymus or Heucheras which sound quite appealing and also consider evergreen ferns. I'm not going to give up!