Oops, sorry for not replying, I was going to but forgot
I think considering what the course consists of, a more essay based subject would be useful. Like Pedro says, you should mail the admissions people to see what they think. However, I don't think that you're at a massive disadvantage.
I think there's some key questions you should adress in your personal statement next year including:
What is it that drew you to consider a Japanese degree? What sort of interests do you have in relation to Japan? Do you do anything in your own time like read Japanese literature, take language classes outside of school etc.?
If you feel that you're at a disadvantage, then you should create a PS that highly stresses your interest in Japanese. I had a similar problem when considering my application because as far as language qualifications go, I only had a C in GCSE French. If you would like, I can PM you my PS. (I didn't apply to Manchester, but I got offers from Sheffield and SOAS)
If you really want to apply to Japanese, feel that you're being held back without arts subjects and don't mind some extra work, you could consider asking your school if you can take an AS in something like History or English next year.
AND/OR
If your school offers the EPQ, you can also do that and center your topic on something Japanese. I myself did a short story project in the style of haruki Murakami, but I'd probably recommend something a bit more academic
The great thing about it is that you can talk about it in your PS.
Hope that was helpful, if you have any extra questions ask away.