I’m going to chime in and suggest London.
However, do understand that on Christmas Day public transport is shut down. No busses, no Underground, no trains. On Boxing Day public transport is also limited.
Christmas is a day to be spent with family. A great day to explore the city on foot ; enjoy a walk along the Thames path or a meander about Hyde Park or Regents Park depending on where you’ve chosen to stay. Having a large park near your accommodation helps with keeping the children busy. Over the years on Christmas Day I’ve loved watching families out and about wearing new coats, hats, gloves, jumpers and scarves. Children playing with remote controlled toys, walking new puppies, and trying out toy sail boats and new bikes. As the vast majority of restaurants and cafes will be closed on Christmas Day you’ll need to make reservations at places that choose to stay open ( some pubs offer a special Xmas day fare) and/or do your food shopping on Christmas Eve day to stock up. Investigate staying at Premiere Inns as some serve food on Christmas Day or an AirBnB offering with a full kitchen.
Christmas lights, window displays, Winter Wonderland in Hyde Park, pantos, Christmas markets and Santa Runs are all part of Christmas in London. The Queen also tapes a message of hope and good will that airs on Christmas Day.
Last year after a lengthy early morning walk through Richmond Park’s Petersham Meadows and along the Thames path I throughly enjoyed watching the 1951 feature film Scrooge ( known in the states as A Christmas Carol) with Alastair Sims in the comfort of my hotel room.
I’ve spent a number of Christmases in London. Each enjoyable with wonderful memories: being totally surprised by a Santa Run in Victoria park, midnight Christmas Eve mass in St Paul’s, purchasing boxed Christmas Cards to help charities, the Classical Spectacular at Royal Albert Hall, a Panto at Wilton’s Music Hall, buskers performing dressed as elves or Santa, the Christmas tree in Trafalgar Square, the unique Liberty department store, the size of Selfridges shoe department, hot ciders, the free and fun Hyde Park Winter Wonderland and watching the Peter Pan cup race in the Serpentine.
Spitalfields and Portobello markets are large and fun to meander about especially if you go early to avoid the masses....check to see what might be special for during the Christmas holidays at the London Zoo and if you are ice skaters the rink outside the Natural History Museum is a must.
I vote for London.