As an English-speaking family looking a language-rich environment for dc, I have visited many French bilingual primaries in London. Comparing to London prep schools, the facilities are shockingly poor and curriculum merely mirroring the French national one without much "enrichment" except English instructions.
Many simply look like an otherwise French public school with more English teaching, transported to London. The classrooms look small and basic and crammed, music room poorly equipped, not much drama etc to speak of. With a slower paced curriculum, less-ambitious student body, more holidays, pupils should be academically behind prep school pupils by 2 years at age 10, while also losing out on the extracurriculars available in good preps. Seem too much to sacrifice for French fluency.
Why any non-French families would pay £20000 for that? No wonder many French families in South Kensington send to preps instead.