They book it by 39 weeks so that hopefully you don't go into labor before it starts. Any cesarean done during labor is considered an emergency and they want to avoid an emergency.
A repeat cesarean mostly depends on why and how you had the first one. If it was for a complication that is likely to happen again, then you'll probably be encouraged to have another one. Also, some emergency cesareans are done with a scar that goes up and down instead of side to side. An up and down scar is not generally considered safe to attempt a vbac with.
I see that you're in the UK. I'm in the US so procedures will be different, but here it also depends on your location in the country as to whether or not they allow vbacs. Many hospitals liability insurance won't allow them to allow vbacs, no matter what. So sometimes, if our local hospital says no vbac, it must be another cesarean, we can drive to the one in the next town and have the vbac.
From what I understand, in the UK most places encourage the vbac but here is it very different. There is this huge fear built up around it even though statistically, it's safer to vbac (unless there is an underlying complication) than to go under the knife again.