They say "Merry Christmas"
It could either be "Merry Christmas" or "Happy Christmas."
In the UK it is usually: "Merry Christmas and a Happy New Year!"
The UK is a country, but it doesn't have its own language per say. Depending on what region of the UK you might say it with an accent or with slightly different vernacular, but on the whole, Merry Christmas would be fine.There are differences between American and British English but we both say Merry Christmas.
They say Merry Christmas in Britain.