my daughter is struggling to get the help she needs as a student with learning difficulties
Sorry to sound awkward, but as far as hard evidence goes, so far it's only your opinion that she has learning difficulties. To determine whether she has or not, she needs to be referred, probably by your GP and for them to do that, you need to provide as much evidence as you can of how and why she is different to her peers.
Also, several other conditions also have similar traits to what most people think ADHD is, (can't keep still and can't concentrate for long) so don't only request an assessment for ADHD, request a referral for a comprehensive multi-disciplinary assessment of your DD's educational needs.
By all means ask her Primary school for a letter detailing her needs there, but more importantly, you ideally need support from her secondary school. You can ask for GP referral without school's support, but sometimes GP's won't refer if school aren't on board and all you're left with then is the option to go private. If that happens, post again for specific advice.
First things first, see the school SENCO and ask if s/he has any concerns about your DD's behaviour, coping abilities, executive function etc. Ask SENCO to ask the teachers for their comments on her behaviour and the standard of her work in comparison to her peers and if they see anything that warrants investigating, ask for a letter to take to your GP.
Then book a GP appt on your own to discuss the GP referring your DD for assessment.
Depending on the area you live in, the NHS diagnostic process for many neurodiverse conditions is running at around 18 months to 2 years.
Exit passes - it depends whether the school only issue exit passes for pupils with diagnoses and reports which recommend them, or whether they issue them at their own discretion. Ask SENCO.
The whole process of assessment, diagnosis, individual recommendations and EHCP which will see a young person through secondary and Uni can be a long-drawn-out one, so the sooner you can get started the better.