Following conversation here, I hit the Perseverance last night, on Lamb's Conduit. Everything I'd read suggested that the beer was poor but the food superlative.
I agree about the beer being poor. Aside from Guinness, the darkest beer was ordinary Leffe (which, if you haven't had it, isn't saying much). At home we call this the Los Angeles Beer Curse: 20 lagers and a Guinness. The Perseverance did have Old Speckled Hen in a bottle, though (along with some other odd bottles -- Heineken, Bud, Sapporo).
I was expecting the food to be more ambitious than it was, frankly. The menu was pretty basic: Spag bol, bangers & mash, lasagna, plaice & peas, that kind of thing. Plus a whole selection of sarnies (although they spelled it "sarnries" -- a sandwich made by Sanrio?) that were essentially the mains on bread. They did have bacon butties, but no chip butty.
On special were pies -- beef, mushroom & Guinness or chicken, leek, & Guinness. I wasn't sure what Guinness would do to a leek, so I had the beef. It was perfectly fine, if a little indifferent. But the chips were excellent!
I think next time I eat on Lamb's Conduit, it will be at the Lamb, where I'll get top-notch beers. The food's unlikely to be more than a half-step down from the Perseverance's.
Oh, ambience: Just right. A nice mix of people, not too crowded, not too empty, no "Look, a stranger" vibe.