Hey, a lady on Twitter (@teawithonesugar) posted this in response to your question - hope it helps! "Rambert Dance will be holding ballet classes 2014 at their new site on South Bank with some tailored for more mature people"
Also, have you tried the beginner's 6 week course at the Basement Dance Studio in Holloway? Sounds really good and very informative: http://www.wahanda.com/service/375536-the-ballet-surgery-6-lessons-over-6-weeks-at-the-basement-dance-studio/
We'll keep our feelers out and let you know if we hear of any more fab places :)