It's not exactly the same, but I'm doing Chinese from scratch at uni. I agree that learning the characters is hard, but it's in no way impossible.
Have you thought about taking a gap year and spending some time in Japan (e.g. working, studying in a Japanese uni), so you'll have a little head start before you go to uni? The only problem with this is that you may have to "un-learn" some things when you actually go to university, but on the other hand you'll know for sure whether Japanese is what you actually want to do.
Also, it's VERY important to study at least one language at A-level (or equivalent). There's a huge gap between A-level languages and degree level courses, and it would be even harder to bridge that gap with only a GCSE in a language.
Another thing - check the content of courses very carefully, because some are more literature based than others. It all depends on what you want in a course, really. Do you want to be fantastic at spoken Japanese, but have little knowledge of Japan's classical literature? Do you want to know everything there is to know about ancient history, but know little about modern social trends in Japan? Obviously, many courses will cover all of these things briefly, but the compulsory units may be heavily weighted towards one or two topics.
One last thing - be aware that language courses at uni (any language, but particularly languages you start from scratch) move QUICKLY. The idea is to get you exposed to as much as possible as soon as possible. At school you may spend months learning one grammatical point. At uni level, it may be covered in a week.