In my day you had a final year submission deadline which was immovable (April/May time of final year) but when you started your dissertation was really up to you.
However, DS has already had to submit a brief overview of what he's planning for his dissertation, so I suspect there's more of a push to get students 'on the case' sooner rather than later nowadays?
Things may have changed a bit since then though! When I did my dissertation, I recall very little input/advice from my designated tutor. That wasn't my choice, it just wasn't the way things seemed to be done then!
I would generally say the sooner you start the better though, given that there will be other assignments (and possibly exams even) to do too. You don't want to be left with loads to complete in a very short time-frame! And cramming for exams to do too.
I would have thought it's better to have the input at the beginning (when one might be struggling more to fine tune the focus of the dissertation). You'd like to think that if someone went with an autumn-available academic, the young person would be well into writing it by Christmas (you tend to quickly gather momentum, particularly if you're really into the focus of study), and therefore would have received appropriate guidance about what needed to be further honed. So they'd have a clear idea by then of what was required to complete the piece of work to a very good standard?
Is there no-one suitable available for the autumn and spring terms though?