Is my foreign diagnosis for ADHD valid in the UK? Also, should I get a private diagnosis locally?

I've got a foreign diagnosis for ADHD and I've got proof of it. I've been told it's useless when it comes to collecting benefits but, I've still used to collect benefits alongside other information such as sick notes; this makes it difficult for me to know if it was actually helpful in getting my benefits. I just want to know to what extend my foreign diagnosis is important or if it's completely irrelevant.

If my foreign diagnosis is completely useless here( in the UK), should I be worried about it? Should I just rely purely on the sick notes?

Also. Could getting a private diagnosis locally help me to calm benefits? Wouldn't it also be taken by the NHS,UC,etc? Is there any problems with getting a private diagnosis that I should worry about? I heard I've been placed on a waiting list but, I'm impatient.

Quoting and linking government sources would be greatly appreciated.