I can't help you specifically but I know how universities are like in the UK. If you're going to be doing engineering as a degree you're going to need to be fluent with A Level maths concepts because that's going to be absolutely standard in your course. If you need maths for a course that means you're going to be using it, a lot. BTEC engineering will simply not prepare you for what you'll be expected to do as standard for your assignments and exams and I don't think this maths entrance exam is going to be easy. I'd definitely switch.