Here's what I get, taking your recipe, adding a pound of light DME, making it a full boil (I used 6 gallons as a boil size and 5.25 gallons as a final batch size), and keeping the hop schedule the same:
Beer: - Style: English Brown
Type: Extract w/grain Size: 5.25 gallons
Color:
25 HCU (~14 SRM)
Bitterness: 21 IBU
OG: 1.050 FG: 1.013
Alcohol: 4.8% v/v (3.8% w/w)
Grain: 8 oz. British crystal 50-60L
Boil: minutes SG 1.044 6 gallons
3.3 lb. Amber malt extract
1 lb. Light dry malt extract
2 lb. Amber dry malt extract
Hops: 1 oz. Willamette (5% AA, 60 min.) (pellets)
.5 oz. Willamette (5% AA, 5 min.) (pellets)
The OG of 50 is the high end of the British Brown style. The 21 IBUs is in the middle, which guessing from your comments, that's what you're looking for (a distinctly not bitter version of the style.)
The calculated color is a touch light for the style, but my experience tells me that a beer made with mostly amber malt extract is going to be a little darker than what these calculations indicate.