Flares may not be familiar to many people, though I expect that so long as you set it off safely (in a large garden, or field) you should be able to do it openly without getting any unwanted attention (often random people watch, sometimes applause, from my experience!)
Even if you are setting off larger fireworks, if it looks like a professional firework, people will assume it is, and they should not have a problem with it, so long as you are being sensible.
Deciding what you can and cannot set off in your garden is up to you, since you know the size of it, and your neighbors/family/flatmates better than us. I personally have good neighbors, and fountains and micro-mines are completely tolerated. On a few occasions we have had a joint early November party and small shells have been fired.
It is better to be cautious than not.
Once I went to a restaurant where they served birthday cakes with some sort of gerb (spat short lived sparks) stuck in, I was quite astounded as to how they managed to produce a few wisps of smoke and no smell whatsoever.
Sounds like Nitrocellulose and Titanium to me. Many indoor pyro is based on Nitrocellulose, since it is completely smoke-free when burning (though effect-giving additives add a little smoke.
Due to cost, and some practical difficulties, it's rarely used in outdoor stuff.
