It's a traditional accompaniment to roast beef, but the Brits make it more like popovers with good greasy fat on top.
Same simple recipe, but Americans put the goo right into the pan under the roast beef shortly before it has done cooking. Better, in my view. Greasier.
Yes, Roast beef (prime rib, boneless or not) is a Christmas Day thing. I've always liked mashed rutabaga with it all, and horseradish with the roast beef.
We'll be having Christmas in olde Albion, but an early one here on the 21st. With Roast beef of course.