Thanks for your patience while I looked into this for you.
You are correct in saying that over the Christmas period (14th December - 5th January) Sky Sports Darts will replace Sky Sports F1 while F1 is in their off season. Motor fans will still be able to enjoy F1 content across other Sky Sports channels during this timeframe. As the F1 season will have already came to a close it means that F1 fans will not miss out on any live races. It also means that there will be no disruption to any love sport on any of the other Sport channels.
In the New Year Sky Sports F1 will be back on channel 407, and will be the only place to see all 20 practice and qualifying sessions as well as every live race. Sky Sports F1 will continue to be a dedicated motor sport channel during the whole F1 season including preseason testing, and every race weekend live.
I have also queried jonny-b's question regarding the channel being free for HD customers, and will respond once I have an update on that.
I hope this has answered your question, please don't hesitate to contact us again if you have any further questions or queries.