There's essentially no such thing as pure democracy at a national level. Even in cases of public referenda and ballot initiatives there's still a small group of (usually) professional bureaucrats of one variety or another that controls what questions are asked, how they are asked, and to whom.
Asking "the people to have their voice heard" on international organizations and trade agreements is particularly ill advised because those are "how" questions instead of "what" questions, and to a first approximation nobody actually cares about how. People care about whether their job is secure, or their groceries are affordable, or their culture is preserved.
Leaving the EU may or may not be a good idea for Britan, but crucially very few people really know whether it would or not. The level of specialized training necessary to reasonably predict the effects of leaving on any of the core interests that people do actually care about is prohibitively high. Directly asking whether to leave the EU is a bad idea because it doesn't capture what "the people" really want and it forces a specific course of action even when there may be safer and more effective methods for achieving the same ends.