How do you know you have PCOS?

Yep, make an appointment with your GP. They will chat with you about your symptoms, and then discuss any next steps/further testing. NHS care is free if you are a permanent UK resident, but you would have to pay for prescriptions (pretty sure it's £9.90), unless you meet exemption criteria.

how about blood test or ultrasound sound arranged by NHS? Do I need to pay by myself? Thank you

So I first went when I was a late teenager as I was having very long period cycles (like 50 days +), the only way I actually knew this was to keep a record (I used flo at the time but this was years ago I dont know what its like now) of when they were happening. I would suggest doing similar and showing it to your GP.
I also had other early signs that I didn't know were PCOS (struggles with weight, excess hair growth and acne can all be PCOS related).
They sent me for blood tests (they check for things like high levels of androgens) and an external ultrasound. The three criteria are increased levels of androgens, absent or irregular periods and 'cysts' on the ovaries (apparently these are not real cysts but follicles). You only need 2/3 to be diagnosed I believe.
Whether I just got unlucky or am more severely affected I dont know but from personal experience I have found it quite hard to manage especially with age and I have a lot of symptoms/side effects. You may be referred to an endocrinologist but the waits are quite long.
I do hope you get the answers you need, I dont mind answering any questions you may have

may I know how long does it take for the gp to refer you to have the blood test and the external ultrasound?