The answer was given the last time you asked question. Black tie is not compulsory but why not join the flow, you may find you enjoy it. With Dinner suits available from Matalan, Tesco, Asda for around fifty quid, it's the cost of a couple of rounds of drinks!
On Fred Olsen ships there are usually four formal nights on a two week cruise. Fred's guidelines state 'On Formal evenings, gentleman will prefer to wear 'black tie' with ladies elegantly dressed in cocktail or evening dress.' You will find that most men wear black tie, or a dark suit, some even in kilts. Ladies are usually dressed to impress. If you really don't wish to dress up you could eat in the 'Secret Garden Cafe', an alternative to the main restaurants where they occasionally have speciality evenings. Perhaps you should consider another less formal cruise line, as most people comply with the dress code.
Some men did not go to the formal nights as they didn't want to dress up .The informal nights still require smart dress. There are some theme nights which you can take part in if you want-Ship supplied some scraves etc .Air conditioning makes show area very cool at shows in evening for ladies.