For one, that port would for the remainder of the period during which Bismarck is placed there going to be the target for Bomber command, commando raids a'la St. Nazaire and Trondheim raid and Taranto style raids by FAA. Until any of these succeed in sinking Bismarck, thre battleships of RN will be permanently based within interception distance, probably in some port in UK, which gives six high class vessels (including those needed for Tirpitz) immobilized at least until US comes into war and probably 12 to 18 subs to be permanently stationed in vicinity.
What effects this has on Med, Pacific and other theaters can be disputed. However, I doubt Bismarck will attempt something like what the twins pulled off, or if she do, she will be sunk. I doubt it will significantly change the outcome, as by 1942, US will bring some of their ships. Germans also might waste some more resources on their surface ships, leading to less u-boats available? And yes, PoW will be saved.