@Erik the Anglophile What yeast are you using? One of the purported Fullers (Pub is my favorite)?
I've done a lot of SMASHY yeast offs' of different English yeasts. That was interesting, but even more interesting was when I did a few yeast offs' against the London Porter recipe below. I was shocked to figure out that the yeasts I preferred in a simple SMASH were quite different in a Porter. WLP085* (WLP002 + WLP007) completely stripped out the chocolate taste. S-04 was the hands down winner in that match up. Suggest you might try a split batch to see what tickles your palate, and what that might mean for your recipe tweaking.
Digression alert: Whilst it seems that brown ale defines London Porters, brown ale isn't for my palate. That same brown ale taste comes through in Samuel Smiths porter and a few other imports I've tried.
Here's a BYO article on 5 British Ale Clones. Whilst this is written by American Chris Colby and does not contain brown ale, I really like his Fuller's London Porter Clone. Nicely balanced with layer flavor. Here's the grain bill:
9 lb. 12 oz. (4.4 kg) British 2-row pale ale malt
14 oz. (0.40 kg) crystal malt (60 °L)
7.0 oz. (0.20 kg) chocolate malt
7.0 oz. (0.20 kg) black patent malt
4.0 oz. (0.11 kg) roasted barley (500 °L)
*WLP085. White Labs won't admit to anything beyond WLP002 and another British yeast. Most wags are for either WLP006 or WLP007. On the plane yesterday back from LA (after a visit to the WL tasting room on Friday), I noticed this nugget in the White Labs
Guide to blending yeast strains: Low Attenuation Yeast ◦ WLP002 English Ale Yeast and WLP007 Dry English Ale Yeast together to get the esters and higher attenuation that you want.