Sorry OP it is so shit. No way should kids be left 2 years FFS how does that help them?
The real big benefit to a diagnosis is medication - in the meantime I would recommend the following.
For you: Watch the videos on youtube called "Russell Barkley 30 essential ideas for parents to know about ADHD" (something like that) - actually, you could watch this with her together, although it can be a bit overwhelming.
Read and/or otherwise access the resources on Lives in the Balance by Ross Greene. Raising Human Beings or Lost At School are probably your two most fitting ones (read one or the other) and the facebook group The B Team. If you can get your head around the Collaborative Problem Solving process, it will enable you to get support put in place for her without any specific diagnosis.
For her: Get her the book "You mean I'm not lazy, stupid or crazy?" It's a brilliant primer and very accessible and jokey.
Also, I have been working through Thriving With ADHD Workbook for Teens (Allison Tyler) with DS1, who doesn't take medication at the moment. I scrutinised every workbook on the market and this seemed like the best one. It's helpful as it's a kind of step by step and helps with coping/management strategies, it is American, but there aren't that many differences except for driving earlier (we will just skip those pages) and the school system is a bit different, but I think it still applies. We read it together which allows us to bring up discussion points that we can then research - I am a few years ahead as got my diagnosis first, but if she's already using TikTok for research then it might be you can look together. These will also give you some pointers, as his doctor (we're not in the UK) suggested to make a note of when a strategy isn't working, as it can be that medication helps the strategies to work. Of course it will be out of date in 2 years' time but it will be a record of things that you have tried and how they went.
If she likes TikTok, I would encourage her to look for the How To ADHD community. It's a similar accessible, lively format to Tiktok but it's really well researched and evidenced - there is a LOT of rubbish about ADHD on tiktok, so warn her that it's fine for a supportive community/funny memes etc but if she wants serious information, there are better places to go.
Overall - it might be that you shift your focus a little bit. Rather than worrying about whether she gets perfect GCSEs or is reaching "normal" milestones in terms of independence in study, do whatever you need to do to get her through the next 2 years mental health intact, and keep in mind that anything long term can be re-done. It's not the end of the world if she gets poor GCSE results. There might be a longer, slower route to do what she wants to do in life if the "standard" route is too stressful or too difficult.