We'd need to know a little more about your preferences - village, small town etc to advise you on location.
All children sit the Secondary Transfer Test early in Year 6 and, to get a GS place, the score needs to be 121 or more. Places are then allocated on distance so you do need to be in Bucks by then if you have a particular GS in mind. For example, Dr Challoners GS in Amersham require you to be in catchment by April of Year 5 and others by early in October.
You need to look at the alternatives should the qualifying score not be reached - that might influence your move too. There are Upper schools [this is what Bucks calls Secondary Modern schools] that are better than others.http://www.buckscc.gov.uk/education/sch ... y-schools/
Also look at the stickies in the Bucks section.