The main reason for the use of ventile in extreme cold is that it remains breathable (gortex and other similar materials are not breathable below about -40 deg C), and does not weaken like some man-made fibres at these temperatures. It is best for dry and cold conditions. Also, ventile is not completely waterproof but for most conditions in the UK, it will prove more than adequate. If you fall into water, it looses all insulation and will act like a cotton shirt!
I have had a single layer smock from west winds for quite a few years now and have never had a problem (I also like the fact that it doesn't rustle like some man made fibres and is 100% biodegradable). The only regret I have is not going for one with a zip all the way down the front, my one has to be put on over the head which is a pain).
Looking at the web site: I can see no real reason for having a cotton shirt made from ventile because it seems to make a better outer layer (it is fantastic as a "water proof" windbreak: I used to cycle in all weathers for about 3 hrs commuting per day and the ventile jacket did not let me down once (i.e. I never got wet)). I might see a use for trousers but as you can buy a cheap quick drying trouser from millets/outdoors etc, I still don't see the point. My legs are not a part of the body where I feel the cold badly anyway, and they are the things that need to keep the coolest when on the move.
I now generally wear a mixture 2 or three of the following: a polar neck long sleeve cotton shirt or base layer, very thick polar neck wooly pully, a thin fleece, and a ventile jacket. This has now proved comfortable for all temperatures I have encountered in the UK.