He wouldn't be migrating mid-install. He would just be cancelling the FTTC install. The question that then remains is whether he could keep the recently installed Sky phone line on an SMPF basis and order FTTC from elsewhere.
I believe he could, as long as some Sky twit didn't cease the line at the same time as the FTTC order cancellation. However I too have expressed reservations about the timescale.
I've got a lot of ground to cover while I've been away!
Today I received a call from the Sky TV executive (CEO's) team and had a 15 minute talk about my order. She apologised very politely, and I explained my frustration, but the outcome is still 7th January. Her explanation was that if they meddle with the dates, they might muck something up and it could get pushed back. In thinking about this, I was about to say that she try to play around with the dates and maybe I'll get lucky, and if I didn't, I would just cancel the order and start fresh with PlusNet.
Why would I do that? I placed an order for ADSL2+ with PN last night for a relative, and they'll be activating that on 1/1/13. The ordering process was a pleasure and I all information easily available to me, and all choices on the phone line/broadband package shown. I also happen to check how the FTTC ordering thing goes, and it provided me with a time slot (10/1/13) but because it's a matter of urgency I went with ADSL2+ for the time being.
The Sky Fibre Unlimited Pro order on the website was almost on the same scale, but I found it less informative and pleasant than the PN one. Now, we all know just how badly the installation went. I can only speculate if it would have been better with PN - I can assume it would have seeing so much positive feedback for PN and negative for Sky - but I am not cancelling my order and will stick it out, I've spent enough time on this for the moment!
With regards to mobile broadband, I happen to have got my Lumia 920 on EE on Monday with a 1GB data allowance. Through some form of OS trickery, or the speed testing app getting stuck at 54Mbit download, I used more than 800MB of the allowance in 3 days! I would only have been happy if my old Galaxy Nexus on Three worked, but the network has 0% coverage in this new area, so I had to cancel my contract with them - I loved Unlimited Data and got it specifically for situations like this where I had no broadband!