OK I clicked on your beersmith link, and your instructions. Maris Otter is not Amber LME, and Amber Malt is not Amber DME. Maris Otter is a base grain, which you shouldn't even have in your recipe for an extract recipe, and Amber Malt is some specialty malt that has no business being there either.
There is an ingredient in the grains which says "Amber Liquid Extract". THAT is what you should be using, along with 2 lbs of "Amber Dry Extract". That at least will get you a more reasonable gravity number. I'm not even sure how you got the number you have...I guess you have some weird setting for efficiency, and you didn't select "Extract" as the recipe type, as you should have, because then you'd get extremely wrong numbers that would make it obvious you were doing something wrong in Beersmith.
But even with all of that, I still get much higher IBUs--in the range of around 35-38. Really you should be using more like .5oz of the Brewer's Gold if you're doing a full boil, assuming they are 8% alpha acid.
So, in the future, if you are doing an extract recipe, select "Extract" as the recipe type. You have selected either "Partial Mash" or "All Grain" because otherwise you'd be getting something like 1.007 for your OG. Then you should substitute Amber Liquid Extract for your Maris Otter, and substitute Amber Dry Extract for your Amber Malt. That should give you IBUs around 37-38. If you click "decrease amount" on the Brewers Gold to bring it down to .5oz, you'll see the IBUs fall into the 20's.
The recipe was written assuming a partial boil, where you add all the extract at the beginning of the boil and you are boiling something like 1/3 to 1/2 of the final volume. Really you could get away with half of the bittering hops (even with a partial boil extract, so long as you don't add all the extract at the beginning) and end up with the right IBUs.