OP, I've been in your shoes. It's a long path so big hugs.
Ask GP for a referral to any other services, or re-refer to CAMHS if harming means lower tier services won't accept. Although even when you do eventually get help, it's a few sessions of CBT which a lot of ND young people simply can't engage with.
Get a lock box for the medication. If she's determined, she'll buy more, but it reduces the impulse option which tends to be strong in ADHD.
Let her stay home from school. Contact safeguarding lead at school. Ask for alternative provision on medical grounds. Don't off-roll her. You want appropriate education to be the LAs problem not yours. The current school is damaging your child's health and is not suitable.
Her overdose is her way of saying she's not being heard. Our daughter now messages the poop emoji if she's feeling scared she might harm herself, and I know to drop everything and take her for a drive - she's safe, the motion calms her and she talks in the car. She also has a sensory first aid kit like a PP mentioned, that tends to work for lower level anxiety not the full blown panic attacks.
Apply for EHCP - it's based on need not diagnosis. You'll likely get rejected first time but keep going. Current school is not appropriate, can't meet need.
Did ADHD assessment rule out autism? There's a big overlap and anxiety tends to be very high in autistic girls especially. The timing of high school is familiar. Can she explain what she struggles with at school? It took my daughter a couple of months to start opening up, but once she could articulate what was hard, it became clear her school would not be able to meet need - look up EBSA as there's a list (or cards) of push and pull factors.
If you can show that you spend a lot of extra time helping her that is not typical for a 14yo, then apply for DLA. It's need not diagnosis, although any assessments help as evidence. It'll take about 4-5 months right now but if approved money is backdated. I took out an interest free credit card, paid for therapy on that, then cleared the balance when the funds came through. I was pretty certain by the time I'd finished the application that she'd qualify at least for lower level if not middle.
Don't panic about education / GCSEs at this point. Her health is more important and there are options (my daughter should be in yr 12, she's instead at a specialist centre taking 5 GCSEs in one year as she couldn't manage to sit anything in yr 11 and had only one term of education in yr 10.)