Apologies for the thread bump but I was looking up North Sea Clothing and ended up here. Ironically I live on the North Sea coast of Suffolk and have found it impossible to buy a decent, traditional fisherman's sweater locally. I picked up a Navy Blue Expedition sweater with shawl collar yesterday from Covent Garden in London. I'd like it for myself, but this one's intended as a gift for a friend. I already have a genuine issue Royal Navy Submariner sweater myself in white, which came from Silvermans in Mile End.
In terms of quality, there's nothing between them (both are made to MoD spec). The RN Submariner is the same weight and 5 gauge knit as the North Sea Clothing one, although a slightly different style as it's a full rollneck instead of shawl collar. Sleeve length is similar on both, with generous roll-up cuffs (nothing worse than the sea breeze going up your sleeve). The RN Submariner is slightly longer in the body and comes down over the hips if you stretch it, which is more traditional - I read that North Sea Clothing made the body length slightly shorter in keeping with a more modern style. But the hem is snug-fitting on both and the Expedition is certainly long enough to cover lower back when on a motorcycle, for example. The Expedition's collar is on the snug side, but there's plenty of give in the wool so it's not too tight, and it'd provide a good option for people who don't want a full rollneck.
The WW2 issue Royal Navy Submariners were made of a very heavy gauge knit, more like a cable-knit fisherman's style. Post war they made it a finer gauge, not quite as tight weave as a Guernsey, but of a similar size. The NSC Expedition appears to be of an identical gauge to the modern-day RN Submariner. Both are snug-fitting. Traditionally these were worn almost skin-tight, to give the best possible insulation, but for a more comfortable feel and better look generally, go a size up. The Expedition is a 42", which is tight on me but the right size for my friend. The RN Submariner is a NATO size 3, which is 44/46", as I found the size 2 just too tight.