I frequently travel as a solo female in escorted groups. I have never looked into all-women's groups. Never really thought to, but I will say that every single trip I have been on had way more women on it than men. One group of about 20 had only 2 men. That's across 6 different tour companies, 15 or so trips.
If you are open to accepting men on the bus with you, I know at least Gate 1 Travel has a London only tour that includes the touristy stuff and free time to do whatever isn't included on your own.
I really think London is doable on your own though, especially if you really aren't looking to leave the city. My friends and I did it no problem just using the Underground and occasional taxi. Never felt unsafe. If I ever had any desire to return to London, I would happily do it on my own as a solo female. I rely on escorted tours mostly when I don't speak the local language or when I want to cover multiple countries in a relatively short amount of time.