They’re used pretty much interchangeably here on the south coast - and it’s been that way all my life, at least in my experience.
I agree that ‘Merry Christmas’ can feel a little bit forced sometimes - and that may be why it’s ‘Happy Christmas’ some of the time. Either works though.
It’s kind of funny how you don’t say anything else is “merry” in a modern context. (Also, only fat people are jolly.)
You’re in Ipswich if I recall… I spent my youth in Hopton (Norfolk) and Feltwell (Suffolk). I don’t think I ever heard “Merry Christmas” unless the greeter was a) American or b) trying to show how culturally aware they were regarding American customs. These were the same folks who would wish us a “Happy Thanksgiving.” (There’s a lot of Brits well immersed in American culture, and have been for generations, especially around Lakenheath/Mildenhall.)
I left the UK in 1986 and I saw a lot more “Americanized” Christmas stuff - Brits tended to have tiny trees, often pink or blue. I started seeing Douglas fir-looking green trees in the mid-80s. Father Christmas typically looked skinnier and had a pointy hat; he started looking like the rotund stereotypical American version at that time. I also thought I saw a greeting card or two with “Merry Christmas” on the front - I noticed it because it was so incongruous.
Globalization sucks sometimes. Last time I was in Britain, half the places felt like American suburbs, with KFCs and McDonald’s littering the landscape 