It was a little bit rowdy, when I was working at Royal Festival Hall yesterday, but that may be because it's school holidays. Even so, you can find a quiet spot - and some power sockets - on Level 2 by the stairs (towards the men's loos!). The Wi-Fi is good! Free, and doesn't require a username or password. All in all, a good day working there. Ooh, and great views too.
Lots of space, a cultural joy!, nice to be in and out, browsing around