I didn't go to high school in the UK, so I can't comment on A-levels. My math background was perfectly sufficient for MST124/5 though. MST124 builds things up from a pretty basic level, so unless you are not confident with basic stuff like algebraic manipulation I'd say be brave and go. I thought MST124 and MST125 were perfectly manageable side-by-side. It's not clear from the website, but they are actually designed to be taken together and the deadlines for the assignments are set in such a way that MST125 assignments are not due that require material that has not yet been covered in MST125. The workload was not trivial to manage, but that's not because it was particularly high, more just because I work full time and had difficulty finding time to study.
It's also worth mentioning that even if you find that you can't keep up with both, the OU has a very forgiving "assessment banking" system that lets you withdraw and take the course again in a later period with very little financial penalty.