Everything in this style points to light balanced touches of a bunch of things-
A touch of nutty malt with the caramel. Light UK hop aroma, with low to no hop flavor.
Medium to medium low hop bitterness. Balance is the key.
I'd move the 10 min addition to flame out, and skip the dry hop addition.
Leave the bittering addition as it is or maybe bump it up a little to hit 28 IBUs total.
Add a small amount of chocolate or light chocolate and back down on the Crystal, splitting it into 2 smaller but different Lovibond crystal additions like Drew said.
One other thing, Golden Promise and Maris Otter are both great base malts. No need for both, though.
I'd do a single infusion mash for this at 151F, and carbonate to roughly 2.75 volums of CO2. (about 5.5 oz instead of the normal 5 oz of priming sugar ought to do it).http://www.bjcp.org/2008styles/style11.php#1c
Aroma: Light, sweet malt aroma with toffee, nutty and/or caramel notes. A light but appealing fresh hop aroma (UK varieties) may also be noticed. A light fruity ester aroma may be evident in these beers, but should not dominate. Very low to no diacetyl.
Appearance: Dark amber to reddish-brown color. Clear. Low to moderate off-white to light tan head.
Flavor: Gentle to moderate malt sweetness, with a nutty, lightly caramelly character and a medium-dry to dry finish. Malt may also have a toasted, biscuity, or toffee-like character. Medium to medium-low bitterness. Malt-hop balance is nearly even, with hop flavor low to none (UK varieties). Some fruity esters can be present; low diacetyl (especially butterscotch) is optional but acceptable.
Mouthfeel: Medium-light to medium body. Medium to medium-high carbonation. Overall Impression: Drier and more hop-oriented that southern English brown ale, with a nutty character rather than caramel.
Comments: English brown ales are generally split into sub-styles along geographic lines.
Ingredients: English mild ale or pale ale malt base with caramel malts. May also have small amounts darker malts (e.g., chocolate) to provide color and the nutty character. English hop varieties are most authentic. Moderate carbonate water.
OG: 1.040 â€“ 1.052
IBUs: 20 â€“ 30
FG: 1.008 â€“ 1.013
SRM: 12 â€“ 22
ABV: 4.2 â€“ 5.4%
Commercial Examples: Newcastle Brown Ale, Samuel Smithâ€™s Nut Brown Ale, Riggwelter Yorkshire Ale, Wychwood Hobgoblin, TrÃ¶egs Rugged Trail Ale, Alesmith Nautical Nut Brown Ale, Avery Ellieâ€™s Brown Ale, Goose Island Nut Brown Ale, Samuel Adams Brown Ale