Glad you said that Talking - I was going to come on to make the same point. Outside Oxford City, Abingdon and Banbury, most of the towns have one school. The towns are mostly about 10 miles apart. So say you designate Wallingford as the Grammar for the south of the County. Do you then start bussing in those who pass from Didcot, Faringdon, Wantage, Thame? And then start bussing those who don't pass out from Wallingford? It would be a nightmare logistically.
As it stands there are constant protests in the press from parents whose free transport is threatened.
Please don't say that it used to work - in most cases the towns mentioned had a small grammar and a small secondary mod (as did Wheatley, Mrs May's alma mater). What has happened over the years is that they have mostly consolidated onto one site, with either the land being sold off and built on, or as in Wheatley, the old Sec Mod being put to other use.