if I still worked at Warbies I could find a demographic, geographic, whateverelseyoufancygraphic break down of double decker buyers from dunnhumby data, but you're just going to have to do your own research.
They definitely do exist and it's not just one big hoax that everyone else was in on. Woolworths seem to be the only stocker of the Cadbury Cream Egg bar though.
But they're a bit disappointing actually. Not sickly enough. You should feel like you want to vomit if you look at another bit of chocolate after eating one. At 46p a time, I'm not sure it's worth it just to have the egg without the foil.