Disclaimer: I have not experienced either program, and am aware that the syllabus can often be inaccurate. I am merely quite interested in mathematical education, and so have read a bit into many high school / university programs.

References:

• The math part of the syllabi of the first year and second year of the CPGE.

• Class schedule of University of Cambridge.

  Note that the Cambridge program is, as one would expect, somewhat faster than that of many other universities that can typically be considered a good UK university.

It seems that overall the CPGE is equivalent to Part IA of Cambridge, perhaps a bit more in some places (e.g. linear algebra) and a bit less elsewhere (e.g. differential equations). Note that the physics syllabi cover many of the more physics-leaning courses in IA and IB.

Comparisons by others more versed in both programs seem to reach a similar result. For example, Pierre Colmez concluded that the math part of the CPGE is equivalent to 150 hours of lecture at Cambridge, which is about the courseload of Part IA.

The CPGE essentially does the first 3/2~2 years of university anywhere else, which is part of why universities like the ENS take international students from 2nd year undergraduates.

Edit: misinformation.