MU123 is actually equivalent to GCSE maths. Meanwhile, most of the MST124 and MST125 material is fairly equivalent to Maths and Further Maths at A-Level. But bear in mind that there is certain material they won't contain. For instance, they won't teach you any statistics (S1 or S2 equivalent), as that can be found in a separate module - M140. Also, you only get minimal mechanics (M1/M2 equivalent, no further).
I can tell you that as somebody who has been studying MST124, MST125 and M140 this academic year and did Maths and Further Maths at A-Level a couple of years ago.
But as somebody else mentioned, you should really contact your uni of choice to make sure they will accept those credits.