You can, and many people do, use 2-core with the armour as the earth, provided that the armour is 'electrically' adequate (which it usually is). Others don't like relying on the (steel) armour as the only earth, because it occasionally happens that the outer sheath gets damaged, allowing water/moisture in, which can eventually destroy the armour by corrosion.
I usually use 3 core if I need the earth at the load end. HoweverI sit very much on the fence.
The difference in price to manufacture 2 pole and 4 pole is negligable, the cost of transporting/storing/managing 2 different versions is significant and would put the price up.
One can over-sleeve any colours (even ones that have never been used in UK) and remain compliant with the regs. Most of us are unhappy with the idea of over-sleeving green/yellow with anything else but (in a multicore cable) even that is actually compliant with regs.
As I implied, few people are particularly happy with doing that. In fact, it's rarely even an option, given the regulatory requirement for a CPC to be run to 'every point and accessory' in an installation - so it can only be done if the 'point or accessory' is getting a CPC from some other cable.
Using the 'bare' CPC as a live conductor is (should be) really a no-no in anyone's eyes, and is obviously non-compliant with regs.
So have I - but that doesn't make it any more sensible or acceptable!
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