While the admissions process for DC with statements is separate from all other DC, actually, in my LA, they 'encourage' you to have chosen a school for the Y5 transition review, and it gets added to the statement in section 4 at that time and the 'new' school is invited to the review (as well as the Y6 review.) My LA had the transition review before Easter in Y5. Other DC don't have to make their preferences for a school until October 31st in Y6, though, so this is generally for the LA's convenience, rather than an enforceable rule.
If you have the school already agreed and named on the statement it makes the admissions process a lot easier (for the LA and the receiving school) because they can include you at the top of the criteria along with all the other non statemented DC, so, despite the fact that schools must take you even if already full, if you are already in the system, they won't have to go over PAN. I went to all the open evenings in the Sept/Oct of Y5 with DS2 then by Y6 only had to take him to his named school, knowing he was guaranteed a place there. Obviously, the open evenings were only a taster of the schools, but it was amazing how many I could rule out from that experience.
The short list left took quite a few more meetings with the SENCos to make my final decision.
In Y6 we went to a transition conference in the March, (when admissions had been set) which all the receiving schools, the primary schools, and the parents of all those DC with statements attended. This was arranged by the advisory teaching service and they had a conference for each area of SN, eg I went to the ASD specific one. We spent 3 hours discussing my DS with the primary SENCo, his primary TA, the secondary SENCo and the secondary lead TA/transition coordinator. The 4 advisory teachers were circulating and we had a detailed questionnaire to fill out for the receiving secondary school. There were 15 DC from my LA included the year I went as a parent, only 7 the next year that I attended as a TA.