Nationality definitions and limitations. Iranian wanting to travel to Japan.

[Arroniran]

Hello,
After a long a winding road I now hold a British passport.

I have searched the forum for my (probably obvious) question but unbelievably can't find the answer.
I was born in Iran but have not been there for more than 10 years and I can't return for several reasons.
I have not and cannot renounce my Iranian citizenship and equally I cannot renew my expired Iranian passport due to the same reasons.

My question is (as silly as it sounds...) what is my nationality?
My passport says "Nationality: British Citizen". From what I can read on gov website my nationality is British but Google and other seemingly credible sources state that nationality cannot be changed. Meaning I am Iranian nationality but British Citizen.

Is this correct? If so, does this affect travel to anywhere?
I belive travel to the USA as a British Citizen/National will be tricky at best in the immiedate future considering my birth place is Iran. Does anyone know how my birth place would travel to Japan? The Japanese visa/embassy website states that 90 days is allowed for those with "British Nationality" which panicked me after reading I am a citizen not a national. Also the Japanese website states that Iranians can only visit Japan if a diplomatic passport. As I am effectively a dual national, could this restrict me? Are there any countires other than the USA I could have issues visiting due to my place of birth?

One more thing to note - my passport has a note on observation pages stating that I have a different name in an expired Iranian passport - not sure if this is relevant.

Thank you in advance for any help anyone can give.

[CR001]

If you naturalised as British on form AN, you are a British citizen, different word for "national".

You cannot change your birth nationality But you can naturalised as a citizen of another country and be a dual citizen with two citizenships.

You are maybe overthinking.

[Arroniran]

Thank you.

Sorry I’m still confused.
So when asked my nationality I still have to say Iranian?

Can I travel the same across the world as a British national (apart from USA)?
Again. Thank you so much.
This is causing me great worry that I could arrive in Japan and be sent back to UK.

[zimba]

You are a dual national so you can choose to enjoy the benefits of the either of your nationalities if you hold the respective passports. So yes, you can travel as British citizen around the world (except for the USA, see below)
It is very simple. As a British citizen with a British passport, you can travel to Japan visa free. The fact that you hold other nationality is irrelevant. Obviously it is up to Japanese authorities to decide to let you in but that has nothing to do with your other nationality.

The only place that your Iranian nationality causes an issue will be the USA where you cannot obtain an online visa waiver even as a British dual citizen. To visit the US, you must instead obtain a visit visa directly from the US embassy which is quite straightforward.