Flexibility (or lack of) at the Open University
I don't have the commenting functionality available, so will have to put this as an answer. 

To be honest, given your background, I think the whole degree will be a waste of your time. I took essential math 1 and 2 two years ago, both are very easy and highly superficial. They claim to teach advanced material but in reality only go through very few and very basic examples in each, so in the end you don't really learn anything. I got like 100% in essential math 1 and 95% in essential math 2 and learned nearly nothing. You'd be better served by buying a good calculus book and studying it. 

And more directly to your question - it is possible to negotiate with them directly. I was allowed to take essential math 1&amp;2 simultaneously because I needed that background for sth else and couldn't wait a year. You just need to email their support team and explain your circumstances.