One of my favorite museums in London is the the V&A. I like the mix of paintings, sculptures, clothing, and historical objects. Many of the exhibits have well-written explanations of how the works were accomplished, often including the original sketches. It's the museum I would go to, if I only had time to go to one.
However, I actually didn't do that many sketches there, relying quite a bit on my camera. It's a great place for gathering reference, and has a long sculpture gallery which contains a Bernini.
I took perhaps a dozen pictures of that Bernini. When I see something that is so clearly superior to the works around it, it really makes me want to throttle those artists who claim that art is subjective, and judging one piece as superior is pointless. All it makes me think is that he or she is unhappy with his/her own work. But rather than strive to do better, it is easier to actually just say the everything else that exists, isn't that great. Yes...art is subjective, but some people are really better at something. You might say that the idea that someone is "good" is subjective too, dependent on cultural standpoint. Yes...of course. I'm sure to an anti-Semite, Hilter was a better person than Mother Theresa.
After a certain point "subjectivity" equals deliberate stupidity.
Ok...onto the Tate!
I didn't really do that many sketches of art at the Tate. It hosts the collection of Pre-Raphaelite paintings that got me through my romantic, nerdy years as a high school and college student., including Waterhouse's Lady of Shallot. 
However, I went there to draw the people looking at the paintings (and escape from the rain, as the original plan had been to sit out and draw people in cafes) Museums are an excellent place to sketch people, as they tend to hold their pose for a bit, giving me a better chance of capturing it. Sometimes, I start with the head and move my way down, but lately, I've been blocking out the general shapes to get the feel of the pose first, and then filling in the details. I've found that if I've been drawing for a bit, I'm generally loose enough to be able finish the details even after the model has shifted (or left the room altogether). Drawing moving people feels like evacuating a burning building, I'm grabbing whatever I can, as quickly as I can, and hope the things I get are important.
This last drawing is from when I went to Oxford on a daytrip. Most of it is a combination of people that I saw while having lunch at the pub and people I saw while having hot chocolate at a cafe.
There is a specific type of face that does look very English, in exactly the same way that someone looks Chinese or African. I noticed it before the first time I went there, but it is funny how obvious this is when you're actually surrounded by them. I suppose it's because culturally, we tend to dump all people of European descent into the category of "white" which doesn't allow for much distinction of features within that group. But French people definitely look French, Italians do look Italian. I realize this is not an original observation at all. I'm not sure if being P.C. has simply beat out our ability to say things like "Hey, that guy has a really Italian face!" However, it's impossible not to see the continuity of features when you observe ancient Roman busts of famous dead people. Some of them definitely look like people you know.