The pots sound a bit small to me, if they're the same as the large tubs I used to get my chicken manure pellets in. And what sort of compost are you using? I prefer to use a soil-based compost in that sort of situation as it dries out less quickly, ensuring more consistent conditions for the roots. Also the heavier weight of soil-based compost gives more stability to large pots which might be useful in your situation.
Having lived in a narrow street, although the front of our house was shielded from the prevailing winds, it was surprising how strong draughts and eddies of cold and drying wind whirl around the streets between houses and they can really effect some plants badly, causing browning of leaves of even quite tough plants.
Of course, the nature of topiary is that the leaves on the 'surface' of the topiary, being from sideshoots are slghtly less mature and more susceptible to scorch than those on the 'surface' of a plant that hasn't been topiarised.
That is why I wonder whether it might be better to grow an evergreen with a naturally fastigiate form rather than one that needs to be regularly clipped to maintain its shape? I think I'd go for a juniper, as they are tough and resistent to wind burn.
There are several to choose from, have a look here http://www.conifer.com.au/narrow_conifers . Go for one of the slower-growing ones
Although the site shown is Australian, most of the plants shown should be available in the UK..