Not strictly speaking self-defence, but you might want to consider the taekwondo club I go to near the Barbican. It's done wonders for my confidence, and in terms of self-defence it has improved my fitness (for running away!) and I'm sure I'd be able to handle myself far better in a crisis now (for example, we do exercices where we are hit or kicked, and hit or kick, with pads, and it gets you over the shock of unexpected contact so you can react calmly).
Just to let you know that we have in the recent past run dedicated self-protection classes here at the Allwell Centre (allwellcentre.com). They are geared specifically towards dealing with the physical and psychological shock of an assault, and are undergoing accreditation by the leading proponents of this approach to self-protection in the world.
We will be running a short one-day introduction to the subject in August, with a view to beginning a new run of foundation classes. If you'ld like me to place you on our mailing list, or just want to have a chat about the field, please do call or write to me.