What is a good salad or vegetable side dish to go with roast beef and Yorkshire pudding?

Celeriac, apple & horseradish mash with Parmesan-baked Parsnips. Good textures and bags of taste. The Gravy is the key.

Brussels sprouts in various preps as noted would be good - very classic English roast dinner. I also always like a simple spinach with roast beef or steak - sauteed in olive oil with garlic, salt and a pinch of crushed red pepper flakes, maybe a squeeze of lemon, to cut the meal's richness. (If you want to dress it up, you could melt anchovy in the olive oil before sauteing the spinach and add a few raisins or currants.)

If you're going to go salad I would do something that can cut through and contrast the richness of the beef and YP. Consider a shaved carrot, radicchio, fennel slaw with apple cider vinaigrette

For any celebration meal involving rich food, I like to serve a crisp, sharp salad. For Thanksgiving this year, I'll be tossing little gems and red butter lettuce with this superb April Bloomfield "Genius" lemon and caper dressing: https://food52.com/recipes/16940-april-bloomfield-s-lemon-caper-dressing with jicama added for crunch and some dried cherries that have been soaked in Moscato, mostly for color - a palate cleanser to balance the cheese, cream, gravy, etc. that will be there en masse. ;o)

This is our Christmas dinner every year, and we gild the lily by adding mashed potatoes....and a big, crisp, vinegar-y green salad and roasted carrots and parsnips. Sometimes we add Brussels sprouts, too.

Roasted brussels sprouts with pancetta is what came first to my mind.

Buttered carrots, pan fried or roasted brussel sprouts and roasted potatoes.

beautiful roasted cauliflower sliced like "brains" brushed with olive oil, garlic & stems of thyme

Like some others, my preference would be to something lighter, acidic and more crisp (the latter to provide a contrast in textures).

A green salad with some crisp/crunchy and/or acidic component(s) that isn't/aren't super dominant, but more of an accent. RIght now I'd consider these fall crops: apples, shaved fennel bulb, almonds/walnuts, late season dry-farmed tomatoes.

For me, a variety of textures is crucial. Roasted veggies would resemble the roast beef and Yorkshire pudding in consistency. I really don't want a nice meal to resemble a TV dinner.

Anyhow, my two cents...

We always have this Christmas Eve with carrot and rutabaga mash. I fear for my lie if I suggest something else!

Amanda Hesser's recent feature salad may be a good match https://food52.com/recipes/9114-kale-salad-with-apples-and-hazelnuts I would also julienne some seasoned carrots for color.

Last night I served roasted vegetables-- green beans with julienned carrots and parsnips--to accompany a Danish pork roast. Sweet and bitter flavors all in one!

I would eat a Caesar Salad with beef any day of the week!

I'm definitely in the something green and acidic. There are several roasted Brussel sprout with balsamic or sherry vinegar floating around.

For salad, a simple Escarole salad would be good. This one is brightly flavored.
http://www.epicurious.com/recipes/food/views/fennel-frisee-and-escarole-salad-351174

haha. i was thinking something on the slightly sweet side to cut the heaviness of the beef and pudding. my s-i-l makes a salad that is topped with pears, toasted crushed walnuts and a bit of crumbled blue cheese and dressed with a citrus balsamic vinegar dressing. it is very mildly sweet and the combo of flavors is really....umami!

Roasted veggies

Broccoli or Brussells Sprouts, something on the bitter side