+1 for herbs in one of the sinks. Oregano, thyme and chives will overwinter whereas basil and coriander you’ll need to grow again and replant new plants next year.
For the other sink I would either do a cut and come again salad trough, dwarf french beans which don’t need supporting or a strawberry trough.
For salad, water the compost in the trough first, scatter lettuce, spinach and rocket seeds (poundland / wilkos / B&M are good for cheap seeds) and then cover with a fine layer of soil. French beans are really easy and give a really good crop, but they will work best if you germinate them indoors and harden them off (take them out on warm days and then in again at night) before planting out. Might be a bit late to get a good strawberry crop this year, but they would put out runners and you can pot those up and plant them again next year.
Re chillies, we also treat them as annuals (no room in the house for them). We have been successful in Liverpool with cayennes outside and now in Cambridgeshire, last year which was fantastic, we got all our varieties fruiting outside (cayenne, jalapeno, big jim numex, cherry bomb). But they all needed support from the wind and the Belfast sinks might not be deep enough.