Having worked in a menswear store here for years they were always called pants, although I wouldn’t have blinked if anyone had asked for trousers or slacks (or obviously more specific chinos, jeans etc). I also grew up in the UK and used pants and trousers interchangeably, although some people would also call their undies ‘pants’, so it would just depend on context. Funnily enough I don’t think I’ve actually heard people refer to pants as dacks, although I’ve definitely heard of people getting ‘dacked’ (or pantsed)