The definition of lewdness may not be obvious, but that doesn't mean that we aren't capable of coming up with a good one to use within the context of the law.
The definition in the context of UK law is simply what a "reasonable person" considers indecent, there is no set definition. The sentencing guidelines talk about "erotic posing" (as the least serious form of indecency, there are obviously more serious forms that are far easier to define).
I'd like to hear your definition, especially with regard to what type of exhibitionist acts a parent can legally convince his young daughter to perform. I don't much care for assertions of what "plenty of people" believe if they aren't attached to explanations as to *why* those people believe that.
My definition of "erotic" would require there to be a sexual element. Nudity is not automatically sexual, there would need to be something about the pose which could be reasonably associated with sex.