i have never been around the north part of m25 on xmas day, in the late afternoon often travelled on the south part again many years ago.
I suspect it may be busier then expected, but if its around dawn, should be OK , I suspect a lot more travel particularly this year , as more people will be travelling to meet up at Christmas while they can, before any lockdown after xmas.
Boxing day on M25 is usually a nightmare, south side J9 around dartford tunnel.
For many years our in-laws travelled from chelmsford to Leatherhead on boxing day, and took them usually 3+hours that morning. And I travelled from Leatherhead to Maidstone around 8am on the day after boxing day and back around 10:30am picking up my mother , and it was always very very busy.