No such thing as a comprehensive in Bucks! If it's not a grammar school, it's an upper school. Comprehensive schools cater for all ability ranges (or are supposed to
), whilst upper schools by definition can't do this as the higher-ability kids have been "creamed off" by the 11+. That's not to say that there are not some very bright children in upper schools, and indeed some such schools are better than others at looking after their more able pupils, it's just that the "top set" won't be the same as a top set in a comprehensive. I personally think that a move from a high-achieving independent school to a state upper would be quite difficult.
Are you near a county boundary with a good comp just the other side of it? That is worth investigating as an option.