From experience there is usually one night per week where people dress to impress but is not compulsory and in any event should be mentioned in your itinerary otherwise smart casual is the phrase we are accustomed to.
cloverjo; You are not alone in misspelling the word, but it isn't "bumph" it should be "bumf", an old R.A.F. term which is an abbreviation of bum-fodder, i.e. toilet paper, denoting unnecessary paper-work :0)
Having done 23 cruises, I can tell you the missus has never taken a cocktail dress. She takes blouses and skirts and mix and matches. That is perfectly adequate.
Cloverjo, Collins and Chambers don't think you are 'misspelling' the word - they cite both. Khandro's explanation of the origin of 'bumf' is perfectly correct.
I've done over 50 cruises and never worn a cocktail dress. Just go with what you'd be comfortable with on a night out at home. Sounds a good cruise, done round Great Britain a few times, Northern Isles are good, enjoy.