It's not really straightforward to obtain medication for ADHD in the UK. Bit of a myth that ADHD medication is widely over prescribed in people who dont have the disorder (perhaps in the US, but not here in the UK).
UK GPs cannot diagnose ADHD (possibly some tiny minority with a special interest and accreditation to do so, but not ordinary GPs). They make an assessment with the patient based on a standardised questionnaire and then decide whether to refer to services that do make ADHD assessments. The wait list for such services on the NHS is something like 2 years. The diagnostic process then involves face to face assessment but they are also looking for evidence from childhood such as school reports and statements from family members etc. Diagnosis is made by psychiatrists or psychologists who specialise in neurodivergence. They may then discuss medication.
You can bypass this process to an extent by seeing a private provider. People are more likely to be doing this because they want to avoid a 2 year wait than because they don't want their diagnosis to be appropriately scrutinised.
If you are then diagnosed and medication is recommended, it can be very difficult to get your GP to issue ongoing prescriptions for ADHD medication. Prescriptions recommended by secondary care which are unlicensed or unfamiliar to GPs require a shared care agreement and in general GPs may not feel adequately compensated or protected for the risk they take on in prescribing +/- monitoring these medications. If you have an NHS diagnosis then at least you can obtain NHS prescriptions from the secondary care provider. But if you only have a private diagnosis you may find yourself in the position of having to pay privately for ongoing prescriptions, which is very expensive.
Lastly there is a notion, or myth really, that ADHD medication is wonderful, improves performance in everyone, has no side effects, is a way for cheating life / exams / you name it. These medications come with side effects (affecting sleep, mood, headaches) and many people have to trial several different ones or can't tolerate them at all. Lots of people with diagnosed ADHD aren't taking medication and behavioural and talking therapies / coping strategies etc are also frequently offered.
Given the state of ADHD assessment it's not that unexpected that some people with traits recognise those traits and implement strategies for coping with them, without a formal diagnosis. I really don't see the harm. People with a self diagnosis cannot access medication for ADHD.