I have seen tons of British, American, Australian kids coming back from CMS to the ferries (mainly Discovery Bay) so guessing there is a lot of English spoken outside of class.
Depending on the age, kids can pick up Chinese BUT...homework, yeah, that can be an issue as you can't really help with that. I see the kids from the Peng Chau school on the ferry going on field trips and the Filipino kids often sit together and speak English with each other.
There is also a school that teaches mostly in English but is designed for learning Chinese- from the names of the kids (they post their work) seems to manly be Nepali kids but some British, American, and Filipino names. It is less well known/popular might be easier to get a spot (because you know about all those issues with primary school spots, I am sure).
By the way, depending on where you work (Central, Sheung Wan, Wan Chai?) Peng Chau is a doable commute and many schools have buses that pick up at the ferry pier. Lots of kids "commute" to school (even kindergarten kids!). It's a nice, healthy lifestyle here, kids riding bikes everywhere, running around and playing, more normal life than other places in HK. The local school teaches English well so even the Chinese kids are pretty good- plus their practice with the Filipino kids. Maybe it wasn't somewhere you were thinking about but check out all the islands area (Mui Wo has a decent-sized Filipino population but haven't seen as many kids at their church as at our church). Basically I am the only non-Filipina at my church but people are very warm and welcoming.