I haven't had anything on the NHS but what I've had would be available, I think. I found a number of things helped me. One was a birth debrief, as mentioned by IsFeidirLiom above. I approached my hospital via PALS for it.
The second was talking therapy: I had 12 sessions with an integrative counsellor through my employer. Not CBT, but it really helped me to process my feelings around what had happened.
The third was using breathing exercises and mindfulness techniques. There's a good website called www.getselfhelp.org, I think, which I like, and which covers the same things my counsellor suggested.
I also found a really helpful book with lots of suggestions for ways to work through your feelings. It's available on Amazon and is called "How to Heal a bad birth: making sense, making peace, moving on" by Melissa Brujin and Debby Gould. Not the cheapest but it really helped me and made me feel less alone. It also helped to have ideas about how to move forwards, as most of the online stuff I found was about symptoms and causes.
The above has taken around four months for me, but means I've gone from daily flashbacks to actually having days where I don't think about the birth. I wouldn't have thought that possible a few months ago. DC is now 14 months, I just wish I'd done something earlier.
If flashbacks are a problem for you, I've heard good things about EMDR but not sure if it's available on the NHS.
Good luck, I hope you find some help.