Hi armandair,
Braschler's Comestibles in the VIADUKT Market have an excellent selection of fresh fish for sushi.
They will even slice it up for you then and there, put it on a plate for you and give you some ginger, wasabi and soy sauce plus chopsticks.
I paid less than 15CHF for a
large plate of mixed tuna and salmon sashimi from them the last time I was there.... YUMMY !!!
After you get your fish for your sushi you can pick up some yummy british cheeses from
THE BRITISH CHEESE CENTRE, some steak and guinness pie from
THE PIE SHOP and if you are there on a Friday or Saturday some lovely shortbread from the
QUEEN OF CAKES.