Worth keeping an eye on https://islington.coordinate.cloud/ for when Islington Council cycle training resumes. Includes 1:1 adult training which might assist with getting used to deal with London traffic. I think the training is stated to resume sometime in August.
If you sign up to the ICC intro ride you'll get to see a decent route out of London as well as learn how to ride safely in a group. You can also cover the latter in Group Skills but that's done on the Inner Circle at Regent's Park so not as useful for helping you plan routes out of London