Sambal tahai is worth adding because it feels like something you actually noticed while travelling through Brunei, rather than a generic souvenir. Tins and sealed products are the easiest version. Fresh or chilled fish is usually not worth the hassle unless you have checked the rules. I would look for it in a proper local supermarket, covered market, deli, pharmacy, gift shop or small producer rather than leaving it until the departure gate. Back in the UK, it is worth checking specialist shops first because they often stock a better version than a broad marketplace listing. The ideal version is modestly priced, not too bulky, and good enough that you would buy it again even without the holiday memory.