So I admit I didn't really think the options fully through when I applied, but I've now received a conditional offer for physics (A*AA, which is the same as the entry requirement for the joint honours with maths) and I'd like to change to mathematics with physics. In my defence my personal statement was for physics and I applied for pure physics at all my other choices. It would have been difficult to fit in a mathematics paragraph in my PS so I probably subconsciously opted for the path of least resistance...
Is it possible to change right now and what do I need to do?