You can very well apply for Actuarial at some places and Maths at others, but the success of this depends on how you incorporate this in your personal statement. A friend of mine got an offer for Actuarial at CASS, however rejected by LSE, an offer for Maths+Stats at UCL, however rejected by Imperial. So it is possible, but I'm sure his rejections had nothing to do with grades (more to do with a passion for a chosen discipline).
I think a degree in Actuarial Science will exempt you from some of the later exams to become qualified (some people like Singh will know more about that). However, in my opinion, if you are fully capable and willing to do pure maths, I would do that. You cannot go wrong - far more respected it is and will keep more doors open. Unless of course you are 100% set on becoming an actuary (from reading some of your previous posts I don’t think you are).