The complicated way to say Merry Christmas in England is by using the English words "Merry Christmas". Quite simple really!!!
Americans say: "Merry Christmas" and people from England say: "Happy Christmas".
Just as in your question !They tend to say "Happy Christmas" instead of "Merry".Merry Christmas
Merry Christmas!
Joyeux Noël.
Feliz Navidad
Joyeux Nöel!
Scrooge initially replied with "Bah, humbug!" whenever someone said "Merry Christmas."
The link has a pretty complete list of how "Merry Christmas" is said in different parts of the world.
"Merry Christmas!" in Italian is "Buon Natale!"
Frohe Weihnachten... merry Christmas
The Merry Christmas song was first sung and performed in the 16th century in England. It was traditionally sung on Christmas Eve as carolers were out and about singing.