Not in your area, but had a look at St Bernard's. There doesn't seem to be a formal 12+, just 'casual' admissions outside of the normal entry point at year 7.
Unfortunately, as far as I can make out, having taken the 11+ but not having taken up a grammar school place would seem to exclude one from getting an in-year place, voles I have completely misread the policy?
- if you took the 11+ and failed, no application possible before sixth form
- if you are wanting to transfer from another grammar school, or
didn't take the 11+ in the first place, and
you pass the test set for the year you want to join, the will think about giving you a placehttp://www.st-bernards.slough.sch.uk/admissions/