I have just signed up a few hours ago and the best date given to me online during signing up (when I would be home) was 9th January.
I had a look in my online account after setting it up and there was an option to change installation date. I looked in there and there were appointments for Wednesday 30th December which suits me better.
Unfortunately, it just keeps saying it doesn’t recognise my order every time I try. I went on web chat to ask but they were pretty unhelpful really. Seems once they have you signed up that’s their job done.