Something doesn't add up here. There is no way she's not gaining weight if she is really eating 50 lbs of a brand name feed. Are you feeding a brand name feed here in TX? If not, there are plenty to pick from but I'd suggest Hi-Pro, Acco, or Martindale.
Since she is bred, I would not use a finisher. We use finisher sometimes on exotic young heifers, but you can get them too fat in a hurry. Almost all bred heifers will get plenty fat without a finisher, especially from a British breed.
Are you sure you are feeding 50 pounds a day? Does she eat by herself or with a group? You mention throwing in some corn and beet pulp? How are you measuring.
Get rid of the beet pulp. It is great for holding, but specifically does not help them gain weight. You can overdo it with the corn. Great for a steer, not a heifer.