I wouldn’t recommend the 12+ unless you live near to a school which historically has vacancies in year 8, or you live very close and could argue a very good case for prejudice.
I am sure you could 'appeal to test' and would stand a good chance of winning. Lots of children move into Bucks after the official test date and sit the exam later in the academic year. I doubt you would be able to sit the test in September, but they may let you take the test at a later date. You could still put the grammar school as your first choice on your CAF form and then potentially if your child qualified before the first / second round of allocations, you could be lucky? I guess you could potentially appeal for a place without having sat the test at all??
Have a look at this page - http://www.buckscc.gov.uk/education/sch ... l/appeals/
You can request an appeal if your child has: been refused grammar school testing (only if the school requires academic criteria for testing)