[somewhatoff]

The slightly snooty Daily Telegraph style guide:

"Christmas lunch is what most of our readers would eat, not Christmas dinner. Use the latter only if referring specifically to an evening meal."

http://www.telegraph.co.uk/topics/about-us/style-book/143530...

[signal11]

(Being slightly facetious) This confirms my suspicion that the Telegraph caters to a very small minority:

https://books.google.com/ngrams/graph?content=Christmas+lunc...

Also this:

https://trends.google.co.uk/trends/explore?q=Christmas%20lun...

[stordoff]

Even in Southern England (Cambridge), I don't think I've ever heard the phrase "Christmas lunch".

[buckminster]

If you're ready for Christmas dinner at lunchtime there was something wrong with Christmas breakfast.