The textbooks weren't written specifically for actuarial students taking the ST6 exam and so there's a danger that by studying these independently from the ST6 CMP you concentrate your time in a sub-optimal way, eg by focussing on areas that are less liekly to be examined and glossing over areas that the examiners see as more important.
Also, the Core Reading consists predominantly of sections of these two textbooks, there is also additional Core Reading that has been written for ST6, which is included and expanded upon in our CMP.
Our CMP is designed to complement the textbooks and guide you through the relevant sections, provideing summaries and extra commentary where useful. It includes extra material compared with Hull in places, to reflect the detail that is likely to be required in exam questions and yet not provided in the textbooks.
However, having said that, if you knew the textbooks inside out, had read the additional Core Reading and attempted every past ST6 exam paper, then I'd be very surprised if you didn't pass the ST6 exam.
But I need to say I'm baised as I'm one of the CMP authors, so it would be good if a few students, past or present, could share their views on going it alone with the textbooks...