Hold off of switching providers until the New Year
Broadband migrations generally take two to three weeks to happen, the absolute fastest it can be is 10 days due to migration code of practice ten day notification period.
This means that if you order a migration today it is not going to happen before the Christmas break but a small possibility that it might happen between Christmas and New Year, i.e. in that magic time of year when everyone is at home a lot more and everyone will be trying to use the broadband even more than usual. The other point to consider is that even if a migration was to take place on 2nd or 3rd January the kids may still be home from school.
Therefore our advice is if ordering a broadband migration to delay the date it happens by not ordering the migation until the usual weekly routines resume in January. Now you could be clever and wait a few more days from today and be sure the migration won't happen until well in January, but we have seen (fortunately rare) cases where the losing provider has caused problems e.g. turning off Internet access a day or two early (the broadband is up and running, but onward access to the wider Internet is blocked).
For those who have new connections already booked, i.e. no broadband now but are expecting to be connected up until the last full working day of 21st December and are planning to be home over Christmas it may be worth making sure your mobile phone plan allows tethering and has sufficient data, just in case the new connection gets delayed or worse looks to be physically installed but is not working.
Update Wednesday 12th December
This advice does not apply to those who have options from two or more different infrastructure providers, since if ordering a new broadband line (does not apply if porting the phone number in many cases) you can overlap the two providers and ensure you have no downtime. We seem some use this as a way of trying a new provider and once happy they cease their old service, though of course this does come at a cost of paying for two services for a short while.