Citibank

It’s got the essentials, but don’t expect anything to happen fast.
Since Citi isn’t one of the big banks in the UK, a lot of their customers in this branch seem to be globe-​trotting expats who usually have
a) a good amount of money
b) an expectation of personalized service
c) fairly complicated questions
This branch often has someone stationed up front who will direct you to the appropriate line or waiting area to get your business done and a couple computers to take care of simple stuff (although if you can accomplish what you need to on a computer, why are you in the branch?). Once you get lined up, things will probably slow down.
To get an account opened here, despite being a Citigold customer in NYC, took 3+ weeks. Each time I stop in, I wait for at least 15 minutes to get anything done, likely due to the type of customers mentioned above and the relatively low level of staffing. Even the one ATM in the lobby often has a line. With that said, everyone has been friendly and helpful, it just seems like Citi hasn’t committed to their London operation strongly enough to spool this place up and streamline things.
Oh, and to ease the wait, there’s a Flavia coffee machine downstairs, nominally in the Citigold area, but I doubt they’d stop anyone if they walked up and made themselves a coffee.