It really depends on when they were in the voyage. The Royal Navy's food was provided by the Victualling Department, which maintained supply depots and victualling yards around the world. Belfast was big enough to have an accounting officer, who was able to purchase supplies from elsewhere to supplement the rations provided by the yards and depots. These rations did include fresh vegetables, and could include fish. Flour was also stored aboard ship, as it tended to be easier to store than bread. Belfast had cold storage, a freezer kept at -10 to -8 degrees C and a cool room kept at -4 to -1 degrees C. The freezer was used for long-term storage of meat and fish, and could be used for vegetables, while the cool room was used for things like bacon. Potatoes, rice and onions were kept in sacks in well-ventilated compartments. Tinned food, mainly corned beef, fish and vegetables, were also stored. Ships tended to eat the freshest food first, leaving tinned food until the fresh food had run out. Even so, fresh vegetables would have been available for most of Belfast's voyages. Fish would have been frozen, but also available. Batter may be a bit of an extravagant use of flour, but fried fish would have been a useful way of boosting the crew's morale - food tended to follow the typical British diet of the time, and crews complained when they had to try new foods.

Batter is not a waste of flour, so long as all the batter ends up on a cooked fish. Frying batter does not render it nutritionally inert.