Oxford or Cambridge? - rare case of very few GCSES

They will both exclude you from the GCSE criteria, in the same way they do for all the overseas educated applicants they get. It's not a big deal.

There are hundreds of overseas applicants who come to the UK to do their A levels and make applications. You'll be assessed on the same criteria as them, the fact you have a UK passport or whatever is irrelevant for this process.

That's not quite the point I was trying to make.
If they decide they'd ignore the language when assessing, the remaining 3 A-level UMS may not be quite competitive enough, as UMS goes, so OP needs to have stronger profile in other parts of application, including good enough performance at interview/test, if he's invited to, to compensate for that.

What would your input be on an application to Oxford instead for the sake of not having to disclose UMS?

That's not quite the point I was trying to make.
If they decide they'd ignore the language when assessing, the remaining 3 A-level UMS may not be quite competitive enough, as UMS goes, so OP needs to have stronger profile in other parts of application, including good enough performance at interview/test, if he's invited to, to compensate for that.

Indeed. Although it could be seen as a bit harsh... after all they won't exclude a candidate's maths marks if their mum is a Maths Prof.

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Having a teacher/lecturer of subject you're doing at A-level is completely different from being brought up in a household with two languages. The way you gain linguistic skills from the earliest stage of your life because of the environment you were born into is very different from learnig maths. Linguistic brain doesn't work like that.

There happened to be an interesting radio programme by Stephen Fry the other day on bilinguals, if you're interested.(don't know which link will work but one of them should)

http://www.bbc.co.uk/programmes/b067wnnb

http://bbc.in/1NGcvdt

Oxford put more focus on GCSE than Cambridge does, so it'd probably depend how they'd view your academic records pre-A levels. If they think your AS results are strong enough and are understanding of your unusual circumstances regarding pre-A levels, you may be invited to interview, but bear in mind you'll have to sit LNAT before interview for Oxford, while for Cambridge you'll sit Cambridge Law Test/College Law Test at interview. I'm not too familiar with oxford's law admission, so idea if they use LNAT result to decide if they invite you to interview or not
As for Cambridge, if they take your Japanese result into acoount in selection, it's definitely good enough to be invited to interview. If they ignore Japanese, average UMS on other subjects are around the borderline level (or just above it) if you're invited to interview or not, depending upon what the strength of other applicants' profiles.

Have you read this yet? (esp. Section 17 & 18)
http://www.law.ox.ac.uk/undergraduate/admitfaqscourse1.php

Am I right in thinking 'a far east asian country' you lived in was Japan? If that's so, it's quite possible they'd (both Cambridge and Oxford) would not take your Japanese A-level result into account for assessment of your application profile, so your high UMS in it may not help you very much.
I know a lot of mixed-marriage families like yours (as ours is as well) around us, and it's quite usual for their kid to sit GCSE/A-level exams of their parent's language a year earlier. They do it because they know they can get good grade even they sit earlier and they can then concentrate on other more vital subjects after that. Having read your circumstances, it sounded like a very typical case of such a family, and both Cambridge and Oxford can tell straightaway that's may be the case, too.

It's a shame that people will most likely presume mixed children learn from early childhood. ( - for domestic reasons speaking Japanese was prohibited in my home)Of course I did end up learning the language by being forced into the environment though, which is an opposite circumstance to non-linguistic learning.

I see. I was slightly more partial to Oxford for their use of the LNAT rather than their own admissions exam, but I will consider my options. Thank you for the link to information on the use of international qualifications.