The problem with going further north in game is partly due to the "flat earth" where SH3 is set. In real life getting to the North Western Approaches via Norway and north of the Shetlands was not such a huge extra distance.
https://www.greatcirclemap.com/?routes=WVN-BGO-KEF
It may also be the real u-boats were afraid of mines. 92083 mines were laid in the Iceland-Faroes-Orkneys Northern barrage in return for maybe 1 u-boat and certainly several merchant ships. The mines seem to have been concentrated around the Faroes, but the Kriegsmarine would not know that.
http://naval-history.net/xGM-Ops-Minelaying.htm
It seems only U63 was lost south of the Shetlands early war
https://www.uboat.net/fates/losses/1940.htm, then a bunch of boats during the 1945 inshore campaign. This tends to add weight to the argument they did not go there on passage.
Going close to Scapa Flow ensures air attacks, but in game and even more in real life these are pretty ineffectual in 1939/40.
With GWX there are lots of merchants close to the Scottish coast, but if I return north of the Shetlands early war I often encounter the entire Home Fleet. This only happens when I have no torpedoes left, how did they code that?