Not all schools have early pre selection. I would be inclined to look at 4 schools, rather than 6. However, now you have 6, keep the appointments. Some may be too far from home. We felt 1.5/2 hours to be the max if you actually want to see your child in performances, sport etc. Some schools welcome parents to lots of things, others are a bit more remote. You can get a feel for this as you go round.
I tend to agree regarding the comments on flexi boarding but unfortunately lots of parents now want this and clamour for change. I think it helps them feel better about sending their child to a boarding school. They come home for the weekend, so they enjoy "quality" time a family life. It spoils the boarding experience for the others though and alters the whole ethos of the school and weekends.
House Mistresses/Masters are so difficult to judge. They can come accross as pleasant caring people, but this is not always reality. Ask the children in the houses for their opinions. Always ask children for their views. They always have them and you do not have to accept their "script" on open days. Probe a bit more. Look beyond the glitz (or "top show" as my Mother would say) and see if the ethos will suit your child.
We found independent schools to be more flexible about GCSEs/A levels than state schools. If your DS is very keen on a certain subject, eg Art, Sciences then check out the teaching and facilities for that. Do they get high flyers in the subjects you are keen on? What are the destinations of the leavers? Have good visits!