Depends what you want out of it.
Reasons to get one: Access to ADHD medication (only available with a diagnosis in the UK). Understanding yourself properly instead of guessing. Workplace adjustments via Access to Work. Reasonable adjustments in education. A concrete answer instead of decades of "am I just lazy."
Reasons you might not bother: If it isn't significantly affecting your life. If you already have setups that work. If you're not interested in medication. If the cost or wait is genuinely off the table right now.
Diagnosis is a tool. It's not compulsory. You don't need one to trust your own experience.