npnp,
You can see the fallacy in your argument that 'the presence of grammar schools makes all schools good' if you think carefully for a minute.
take Kent - a fully selective county. The 25% most able (or best prepared) children go to the grammar schools. Thus the other 75% (including a disproportionate number of those from deprived families, because they are much less likely to have been able to prepare, or pay tutors to prepare, for the exams, and are also more likely to live in areas served by relatively poor primary schools) are in the other schools in the area.
Without any - or very few - of the able students from the area, it is MUCH harder for the 'other' school to do really well. That's the top couple of sets in Maths, for example, or in Science or English. The problems associated with very much higher levels of deprivation also affect such schools in terms of pupil nutrition, poor housing, parents involved with education and having aspirations for their children, involvement of social services and other children's services - all these have direct effects in schools but also indirect effects on staff time and attention. Also, the 'other' schools are generally seen as lower status for staff, and may struggle to attract really good teachers, partly because they may well not have a sixth form, and many teachers who love their subjects want to teach A-level as part of their work.
Even in partially-selective counties, the presence of a grammar pulling away a percentage of the most able, least deprived children of the most involved parents will tend to have a negative effect on surrounding schools - especially as parents tend to compare absolute results, not progress, in schools and so will denigrate the other schools as 'poor' compared with the grammars because of their lower raw results.
So your 'holy grail' of the availability of relatively non-selective grammars (in order to have the greatest chance of getting your children in) and excellent alternative schools is, by definition, hard to find. I also think you are being hugely naive about the need for preparation - even very bright children need training for the exam format, in order to maintain parity with the other children who are taking the 11+ (in muy area at least, a very significant % of children who get to grammar have been to private primaries who teach to the 11+ throughout) and state primaries are in general not allowed to provide such preparation for the exam, which is very early in year 6.
Your best strategy might well be to site yourself right next door to an excellent comprehensive in a partially-selective area (and I really do mean right next door - catchment areas are always smaller than estate agents say) and then enter the 11+ as a 'nice if it happens' alternative. As said above, Gloucestershire away from Gloucester itself would fit the bill.