I'm just another mum of a 15/ 16yr old, so not giving advice, just experience. My DS masked so well, did well academically at school etc, that he wasn't picked up on at school. No GP involvement as health was good,etc.
He deteriorated with behaviour at around 13 or 14. No help from school. Eventually at 15, we decided to get a private diagnosis, as just from on MN etc, I was aware waiting lists were too long and GCSEs were impending. His handwriting is terrible, etc, but school wouldn't help.
We used an online service, Autism Assessment Hub, who do assessments to NHS NICE standards, with a multidisciplinary team, including both face 2 face and zoom appts, etc. We did test for Autism and ADHD, and he was diagnosed with both. We had a clinical psychologist, occupational therapist and speech & language therapist assessing him.
Mostly online seems to be acceptable at this age, and they have therapists available at hubs across the country for the in-person assessment. I considered another similar service too, just happened to go ahead with AAH.
Our NHS GP has so far accepted the in-depth report and diagnosis. We are currently getting medication through a pharmacy online, who partner with the AAH. They do their own assessment too, including zoom calls for each new prescription, but the AAH diagnosis and report are a large part of it.
We are still paying for the prescription privately, but I think if my DS stays settled on this dose for 3 months, they will ask the GP for shared care prescribing. But it is up to the GP to agree, and we aren't at that point yet.
Good luck!