Korean Qualification / CoursesWatch
Cambridge offer an IGCSE but that’s in Korean (First Language) so I’d assume it’s more like an English GCSE than a Foreign Language GCSE.
Your best bet is you’d like proof of your proficiency is to take TOPIK:
That’s the most common proficiency test for Korean. It’s used for things like university-entry for foreigners wanting to study in Korea, and so whilst institutions here might not be very familiar with it, it is an “official” respected test.
The website says it’s valid for 2 years. That doesn’t mean you lose your qualification as such. It just means it likely will not be recognised if you wanted to use it to get a visa or study.
It’s pretty standard with language proficiency tests to have a short validity for immigration purposes - basically because having a test from 5 years ago doesn’t prove you have that proficiency right now as you might never have used the language since.
But you’d still be able to say you achieved TOPIK at X level on X date.
I have a Spanish proficiency certificate that had a similar expiry. I still have the certificate to prove my grade, but I also got a code that I could give to people which could be entered online to authenticate the certificate for 2 years. So I can’t use it for official purposes now. But I can still list it on my CV and it’d be up to employers/unis in the UK whether they accepted it or not if that makes sense!