I admit to being pig-ignorant of UK regulations, but am I reading this thread correctly in understanding that there are two different ratings being discussed, one a full instrument rating and the other something less than that? Perhaps I'm just confused. In the US, there is one instrument rating. It's good for the category(ies) of aircraft you fly, and may be used for either PPL or CPL. An ATPL holder doesn't have a separate instrument rating, because that's assumed and required for the ATP.
I don't have a copy of the UK regs, nor the JARs, nor the interest in reading them, because I don't suffer from insomnia. Could someone briefly explain this issue to a colonial?