Sorry - a bit late here, just got back from a trip.
I flew Liverpool/Geneva with Easy Jet in early March. I had no problems with the empty (and de-valved) cylinders; it wasn't even mentioned. You will find, however that the weight allowances aren't all they could be and you'll end up paying an excess. Compared to the cost of the unit, this isn't much
it also isn't as much as ferry / fuel / extra accommodation combo I wouldn't have thought.
Martin