Well, Marmite is a yeast product and Bovril is a beef extract. There is certainly a difference in flavor, but how much you notice it will depend on how prominent it is meant to be in your recipe. A modest amount of Marmite swapped for the same amount of Bovril in the context of lots of vegetables, beef, and other flavorful ingredients probably would not be earth-shaking.

If Bovril is meant to the basis of the beef stock for your soup, you are likely to notice a difference between your beef stock and a yeast stock, although we have seen yeast extracts described as haring a "beef flavor."

We are intrigued that you are in Texas and swapping one definitive British ingredient for another British ingredient. If it is the beef stock we are talking about here, do yourself a favor and make beef stock or run to the store and buy some. Unless you are British and were raised on Marmite, you will like a beef-stock-based soup more than a Marmite-based soup.

If you are looking for a vegetarian solution (and Marmite fills that bill), you are unlikely to be making prime rib soup….