I just spent a few days in London, and the coffee scene is really vibrant out there, with apparently lots of good shops, though I tried only two because I wanted to try a lot of different things from one shop. I was mostly around the Covent Garden area, so I went to Notes Music & Coffee. They offer espresso, Aeropress, and V60 pourover, each using a different bean. The coffee was in general excellent, with the espresso falling between the blandness of an Intelligentsia shot, and the face-melting shots (Verve shops) that amplify every flavor in the bean. The filter coffee was well-prepared as well, tending towards the lighter side, which fits with the popularity of the lighter roasting style, but the flavors were generally brought out well, and most importantly, in balance with each other. I never had anything bitter. They tends towards acidic and fruit forward with relatively delicate flavor balances. Everything was just so when the extraction went well.
I was really impressed with the knowledge and care of the baristas. For example, they will presoak paper filters, weigh their grounds, measure their water (by volume), and preheat every instrument and holder that comes in contact with coffee. Grinders were generally Dittas for the filter coffees. For Aeropress, they use the upside down config with 12-15 g of grounds, 220 mL of 97.4C water, steep for about 1:30, and push 20-30 seconds, all timed and weighed with precision. Stirring was minimal, almost like just waving your hand over the coffee. Interestingly, Caravan (mentioned below) says that with the right-side up config, one should use a higher dose of grounds: 15 g instead of 12. Notes did 15 in an upside down config, and that's what I do, too. It probably depends on the bean and your tastes.
Aeropress in a commercial environment was pretty popular throughout the city, with another coffee bar I visited (Caravan, near Sadler's Wells theater) having multiple Aeropresses for a big rush. The barista at Caravan apologized and said his main job was as bartender (Caravan is like a hybrid specialty coffee bar and regular drinks bar), but could still tell if something was over- or underextracted --- he was complaining to me about how his V60s usually end up a bit over, and the little sample he gave me was ever so slightly over, but nothing as gross as some of the espressos I've had from specialty shops in the US. The baristas I met were all uniformly very well trained, and knowledgeable. I'd say they were a cut above your average 3rd wave (ie. the specialty hipster places) American barista. They also didn't dress like hipsters, but a popular place that I only walked by (Monmouth) did have a very hipster-looking staff.
Pourovers were a bit simpler: they pour a little in to prebloom, and then filled the cone with the rest of the water. They didn't seem to know about the pulse method, and I suggested to the Caravan barista that perhaps that was why his pourovers were getting overextracted.
The atmosphere at the coffee bar is also very friendly, unlike our somewhat isolated, laptop-reading experience in the US. It feels almost like a traditional English pub. On the two days I went to Notes by myself, I found someone next to me who was really knowledgeable about coffee. They knew coffee at a level that would exceed the knowledge of most US baristas I've met.
Many bars offer at least 2 single-origin coffees for their filter brew. All that I saw used espresso blends.
Most people were also very curious about coffee in America. They all knew about Starbucks, but most had never heard of Peets, much less any of the specialty 3rd wave roasters. One barista did know the west coast coffee people relatively well, but that was because he'd met many of them at an SCAA event or something similar.
For me, the highlight was visiting HR Higgins in Mayfair, thanks to a suggestion from RJman. They are a tea and coffee supplier, and import and roast their own beans. They had some really exotic beans that I have never seen anywhere else: did you know they grow coffee on one of the Galapagos islands? 15.70UKP for 250 grams! That's like $50/pound.
There's also a Malawi Gesha, and monsoon-washed Indian beans. Their selection of Indian beans (most of the staff were Indians) was the biggest I'd ever seen, mostly because almost no one in the US supplies Indian beans! I got to talk to their roaster directly for a long time, and learned a lot of interesting stuff. He was also very curious about American coffee, especially about how the specialty shops sell their beans. He was the only person who had heard of Peet's, but didn't know much about them other than the name. Their roast profile is pretty light, but they also have a few beans they roast very dark.
The HR Higgins cafe downstairs will also prepare a French press (they call it "cafetier") from any bean they sell in the main shop. It's half price, about 1.60UKP for a 125 mL container, if you buy 500 g of beans from them. That's a great deal, and a great way to try different coffees before buying.
Anyway, due to limited baggage room, I came back with only 3 different beans in medium quantities: HR Higgins Colombian Madelin and Ethiopian Longberry single-origins, and a Square Mile Roasters Tanzanian. I've tried the Tanzanian already, and I may have to experiment more as it is a really lightly roasted bean. With my regular preparation methods, which work for American 3rd wave roasters, I get barely sweet, cucumber/green pineapple notes, but without the strong tanginess that's underextraction. I'm not sure if that's what the beans are intended to taste like, or whether I can get more out of them. They are certainly not averse to sweet coffees: the Aeropressed Rwandan I had at Caravan had a really appealing dark plum sweetness.
Costs were not excessive relative to their espresso prices for the filter coffee: 2UKP for a shot of espresso, and maybe 3.5UKP for the filter.
If you're going to London (or live there!), the book I used to find interesting coffee shops is this one: http://www.independentcafes.co.uk/london.html
Their shop delivers to the US pretty quickly.
Also apropos of nothing, I saw a McLaren MP4-12C parked outside Caravan, parallel parked with the other regular cars, slightly dirty, which just made the whole experience that much better. Of course, I had to gawk.