You're right, Wix is a feeder for Bolingbroke, Shaftesbury Park isn't. Wix has been bilingual for a long time, SP is in its 2nd year - that has both pros and cons.
Wix is full to the bursting, with even siblings not getting a place in the bilingual stream, SP (for now) has still got quite a broad catchment because the bilingual stream is new.
Visit both of them if you can and make your choice based on how you feel there. Find out if Wix are doing something like this:
http://www.nappyvalleynet.com/mums/view ... 54&t=60112
Talking to the parents is a great way to get an insider's view.