You can tell the ones which aren't by watching them getting removed in a hurry when the government finally notices too many people using them. 10-15 years back, some colleagues had made businesses for themselves just so they could receive their real jobs' income at the lower rate of dividend income rather than the income tax rate. I am told this is no longer possible.

Conversely there is (or was) what I think was a deliberate loophole for UK inheritance tax — if I remember right (not a lawyer) it works like this: physical objects in your home are all bundled together and valued at £1 for inheritance tax purposes, so fancy art, stamp collections, etc. don't get taxed.