U.K. Warmth Layer
Wool sweater in olive green (i.e. wooly pully). You’re looking for a round neck version with epaulets, shoulder and elbow patches.
You could also go for the much loved “Norgie.” Originals have a towel-like inside the current version (S95) is fleece-like. This piece of clothing has ally points as during this time frame they were only issued to those who had attended arctic training. An important note is that they are cotton, which if wet in cold conditions will provide no warming value therefore it is recommended that you have this in addition to the wool jumper.