How winter changes the day
In London the sun sets before four o'clock through most of December, so by the time a winter wedding reaches the drinks reception it is already dark. That moves the party earlier: the room feels like evening from the meal onwards, and guests who would be on a terrace in June are inside and ready to dance sooner. Plan the music for a longer evening, not a shorter one.
It also changes the room. Winter weddings happen indoors, often in venues with fixed sound limits and firm finish times. Ask the venue for both before you brief the DJ, because they shape the whole plan.
Christmas music, or none
A December wedding is not a Christmas party, and most couples do not want it to sound like one. The usual answer is a light touch: a seasonal record or two during the reception, perhaps one singalong late on, and a party that otherwise sounds like your wedding. Say what you want on your must-play and do-not-play lists, and say it about Christmas music specifically; a good DJ will ask.
Weddings in late November or January have the opposite problem: guests have had a month of Christmas songs and want none. Tell the DJ that too.
What to ask a winter wedding DJ
The questions that matter for any wedding still apply: can you hear a full recorded set, how will they handle a floor that spans generations and cultures, and how do they work speeches, the cake and the first dance into the timeline. For winter, add three: how they would pace a longer evening that starts in the dark, how they would handle Christmas music, and how they plan travel and load-in around winter weather and the December party traffic.
December Saturdays compete with the Christmas party season for the same DJs, so book early. Weddings in January and February sit in the quietest part of the calendar and are usually the easiest dates to secure.