I went to my Adult assessment at 18.5 having been seeing CAMHS/ been in hospital since 14. I was really lucky that a) my psychiatrist kept me until 18.5 and b) came to the assessment with me, saw how awful it was and c) kept me until 19 and 1 month when I left for university.
University (mine at least) enables this sort of 18-22 service in that I see a GP, mental health advisor and then they are arranging me to see an (adult) psychologist but she mainly sees students. Being an university city the crisis team were also really good with understanding the stresses that caused. However there were some services they considered referring me to but didn't because of the age thing, they didn't really want me to end up in a group of much older mentally ill people. Which has it pros and cons.
I am in a lucky situation though, not just with camhs but also that I chose my university largely based on its mental health/medical services. As in, of the places I was headed for grades wise, I liked this one and on the pluses it had good hospitals and services and also a lot of charities that do counselling etc. I don't mean I purely researched unis based on mental health services.
(I'm at Nottingham and it appeared to have better services than Essex, Oxford and Winchester. London has good services but was going to be very based on which bourough my accommodation was in each year).
I think it is a very hard jump. Once your an adult their favorite phrase is 'you have to take responsibility for yourself' (which I interpreted as leave us alone you have to look after yourself, apparently it means attend appointments and take meds - which i was doing anyway, hence the confusion). It also can become very much about immediate risk and if you don't explicitly state that risk they aren't always willing to read in to what you're saying/ how you present. But I'm still alive so maybe that isn't such a bad thing (in my case) I don't know.