The traditional celebration begins on Christmas Eve with stockings (or a big sock or even a bag) being filled with small presents for the children (and sometimes adults too). Officially it's Santa Claus who fills these, so don't forget to leave a mince pie for Santa and a carrot for his reindeer beside the stockings!
In the morning the children open their stockings, the family exchange presents and breakfast is usually quite quick and light (though some people have champagne). For those who go to church on Christmas Day, morning is the usual time, and otherwise the big event is Christmas dinner. Typically you eat this around 2pm.
Turkey or goose with stuffing, roast potatoes and brussels sprouts are the basics, followed by Christmas pudding, but there are endless possiblities. Then in the afternoon people usually go for a walk, play games, listen to the Queen's Speech and maybe watch a favourite TV programme like The Snowman. It's traditional to finish with turkey sandwiches in the evening.
You can find a full traditional Christmas Day menu here.