If you're looking for stats to back your letter up, you can find the Home Office stats for rape and sexual assault in London and rape and sexual assault nationally. The curve for rape is pretty shocking although possibly reflects a rise in reporting rape as much as a rise in rape committed.
There's also a petition on the Downing Street e-petitions site calling for more rapists to be convicted which also mentions Rape Crisis Centres. I may start a petition specifically about Rape Crisis, but it requests that you avoid duplication so I haven't decided whether that's the correct thing to do yet.
You could also contact Harriet Harman, who is currently Minister for Women and Equality.