You need to talk to different people. Insofar as it has a physical cause (insufficient dopamine production), it is a physical condition, but anyone who tells you that there’s no point getting medicated due to your age, or that ADHD is solely defined by excess movement clearly doesn’t know the first thing about ADHD, and thus is not qualified to dispense advice on the topic.
I’d suggest maybe going through your son’s school, or using some of the UK ADHD resources to find out what specialists there are in your area, or even out of your area if you’re able to travel.
In terms of medications, realistically your specialist should be the one to drive the process, but the types of meds you should be looking at have the brand names Ritalin, Adderall, Strattera, Vyvanse, and a few others. They’re either amphetamine medications or methylphenidate medications, depending on which one you take first.
As for support, support is supposed to be tailored to the individual, and the specific symptoms you experience. As you probably know from bimbling around on here, ADHD has three presentations- Hyperactive-impulsive, Inattentive-impulsive, or combined. Your specialist should help you determine which you are (by the sounds of it, you may be the inattentive type, which appears to be the most common in women), and the specific difficulties you face inform what support you need, with medication as a part of that.
I’d suggest starting with the adult self-reporting scale, which you can find here: https://www.google.com/imgres?imgurl=https%3A%2F%2Fs3.amazonaws.com%2Fcdn.freshdesk.com%2Fdata%2Fhelpdesk%2Fattachments%2Fproduction%2F3018251382%2Foriginal%2Fasrs_symptom_checklist.jpg%3F1460753137&imgrefurl=https%3A%2F%2Fhelp.junoemr.com%2Fsupport%2Fsolutions%2Farticles%2F3000051740-appendix-1-adult-adhd-self-report-scale-asrs-v1-1-symptom-checklist&tbnid=xeXUj2OA7OOfCM&vet=1&docid=fEZphJdTwRdChM&w=1129&h=1449&client=ms-android-sonymobile-rev1&ved=2ahUKEwi49rLAxtvfAhVwxIsKHfjMAZ4QMygAegQIARAo
When I had my first appointment in Norway, I’d written out all these questions, and given examples from my daily life, and my past, on how I experience each symptom. Then I translated it to Norwegian (I’m from the UK, too). These notes actually helped save me about 7 appointments for diagnosis purposes. I definitely recommend chasing up with your GP, or even switching GPs if your current one isn’t supporting you. Do you have any friends with adhd, or ADHD kids? If you do, try seeing their doctors and see if you have any better results. Otherwise, keep on at your GP, demand to see an actual specialist, even if you have to go out of area, and correct your GP and anyone other than a specialist (or even your specialist if need be) when they start spouting nonsense. There are dozens upon dozens of resources online, even on the NHSs OWN website. If they bothered to research even a little, they’d find out.
I wish you the best of luck. Any questions, just ask 🙂