Train, plane or car? I suspect that this is one of the few times that driving might win, but as I've worked most of my life in London I've never seen the need to own a car so I can't be certain. But I can imagine there must be some pleasure sitting in your own space not having to worry about the aisles blocked with other people's luggage and worrying about space for your own.
Fortunately for me most of my train trips home for Christmas have been enjoyable ones, it pays to book early and get first class. I also try and travel a day or two earlier so as to miss the last minute rush. Christmas is one of the few times I also travel northbound on a breakfast train, travelling earlier also seems to avoid the crowds. As it's Christmas it's also nice to grab a prosecco from the food bar and have a Bucks Fizz for breakfast (still too early for Hop on Board).
For those who are unable to book ahead (which includes me for the first time this year) things can be a bit more stressful. Standard class is likely to be very busy including with people who haven't been able to reserve a seat. Luggage space is also going to be at a premium. So if you are travelling from London today, even if you have a reservation, it's going to be advisable to get to the station early and be among the first to board. Trains to Edinburgh usually offer the facility to 'check in' luggage to the guards van. If you're going all the way to Edinburgh then it's worth making use of this facility.
Flying has its own set of challenges over Christmas. On a plane you don't have to worry about other people's luggage blocking the aisles or seats but you have to be more careful about what you bring and if you need to check things into the hold you need to be aware of the baggage allowances. You can't take Christmas crackers through security so they can't be in hand luggage and it's advised not to wrap presents in case security needs to inspect them. On the other hand BA have put on an extra flight to Newcastle (BA1328) which still has seats available even a few hours before departure so if you need a seat home at the last minute, this could be a safe option.
So my preference is train in first class if you can plan ahead, if you're unable to book ahead then flying can look more appealing because you know you've got a seat! If there's no seats on the planes and the trains have no reservations then it's a case of getting to the station early and hoping to get an unreserved seat. You need to check online for the platform numbers and get to the platform before it's announced in the station!