I don't know anything about fishing or seafood or business, but not knowing can be a good thing sometimes.
This is not an exhaustive list of possibilities, but I think it'd be a good start.
If anybody would like to employ me to deliver interesting ideas, do contact me. My email address is jasmine(at)ifihadthemoneyidfollowspring(dot)co(dot)uk.

a. Fish. Any white fish that is not over-fished. Menu doesn't state that pollock is sold, nor that haddock is sold. This allows for flexibility based on the availability of fish; fish can be chosen on what looks best that day, or what is abundant, or what is cheapest. The proprietor can even choose a fish based on facts about fish stocks, choosing to stay away from pollock if it seems like stocks are declining. Literature will state this policy, letting the customer know the reasons for such a decision. What would be the reasons for such a decision? To avoid depletion of fish stocks. If every chippy in Great Britain put cod or haddock on their menu, there might not be much cod or haddock left soon. No need to change menu prices every time the fish changes; if people worry that they are being overcharged, simply take the difference and put it towards something good. No farmed fishes. Possibly supplement the menu with other seafood that looks great on the day of visiting the fishery. This'd open up the possibility of doing more than the obvious fish and chips, of including razor clams, say, maybe in the form of a fish pie.

b. Chips. Find a local organic potato producer. Find several. Ask what potatoes they grow. Ask them to provide you with a few of each, and test them. Are they waxy? Is waxy good for chips? Make and eat chips every day to find the perfect potato. Would it be possible to find home growers or allotment holders in the area who'd be interested in supplying you with some potatoes for a few months of the year? They could enjoy free food at your chippy once a week in return for this. Your potatoes would be really local if you did this. If the allotments are all in the same city, collecting the potatoes could be done on a work bicycle with a trailer attached.

c. Other food. Ketchup from local tomatoes. If you want to invest time and effort in making your own ketchup, is there any space to grow some tomatoes on site? Either way, perhaps consider bottling ripe tomatoes when they are in season so that there will be a supply of grown in Great Britain tomatoes for you to use.

d. Furnishing and decoration. Second-hand chairs and tables made from a material that could easily be turned into something else. Maybe wood would be the most obvious choice. Or a local woodworker could be commissioned to make the tables and chairs. Water-based, palant-based paints, free from toxic volatile organic compounds. Perhaps salvaged tiles for the floors? Indoor plants are good for improving indoor air quality. Avoid plastic items for which there is no real need: plastic cutlery for diners, for instance, is unnecessary when metal cutlery exists.

e. Resources. Seek to use a minimal amount of packaging for the food. Ask suppliers if they use reusable delivery containers; if they don't, ask if it is possible for you to provide your own that they will put your stock in to deliver to you. Sell drinks in glass bottles rather than in plastic bottles. Big windows for lots of light pouring in; shun artificial lighting where possible. Avoiding things like sachets of ketchup and other packaged food that can easily be made in house will save creating unnecessary waste. Energy from a renewables energy supplier. Consider the viability of producing your own energy. Compostable packaging for take-away. Use only easily recyclable or reuseable materials and items (items like bottles used to provide tap water, say, could easily be used to bottle ketchup, for instance).

f. Waste. Recycling receptacles on the floor for customers to separate the waste resources as you go: waste food; waste packaging. There will be no receptacle for plastic bottles, because you will not sell plastic bottles, though you could accept any plastic bottles that people near to the shop might need recycling. That is, if somebody happens to be walking past and has an empty plastic bottle that they don't want to thrown in the bin, but don't want to carry all the way home, they could deposit it with you. Any food a customer has left over that will go into the food waste bin, along with potato peel, could be decomposed on site using a bokashi bin; when ready, the contents of the bin can be added to a wormery, with the resulting worm casts mixed in to the growing medium for your tomatoes, or given to an allotment holder. Food that is soon to be waste, yet still uncooked, can be cooked for people who are in need of a meal. There are charities that do this, but perhaps you'd like to invite people over towards the end of the day to consume the food.

Some other ideas.
1. If you find that your chippy is using items that have no obvious further use—let's say you use boxes made from recycled card for people to take away their fish—to save all that cardboard being recycled again, and so soon after its new lease of life, perhaps consider giving away some seeds with every portion of fish. You can say to people that the box could be used as a seed tray, or it could be ideal for growing micro leaves. The seeds you give away would vary according to what season it is; maybe three or four legume seeds in March, a handful of cress seeds at any time of the year. I think it's a great idea.

2. Your staff decide what they would like to waer to work each day, but you could stipulate that whatever they wear on their top half is brightly coloured. Bright colours are excellent.