Unconditional offer advice

Hi guys,
I'm fairly new here and completely new with the UK school system, so I really need some advice. I'm currently in my last year of high school in the Netherlands. We have three different kinds of 'levels' here, I'm doing VWO (which is the highest level). Grades aren't really that important here, it's more the level that you're in that decides which kind of school you're going to. Anyway, I am going to study in the UK next year, mainly because they don't have the course I'm interested in studying here in the Netherlands.

To summarize it all (shortly): Applied to 5 schools through UCAS. Got conditional offers on 4 of them, unconditional from one school. Those conditional offers wanted me to have a 7 on average on VWO, and before my exams I had a 5.9 average. Whoops. And the university I received an unconditional offer from was my #1 choice anyway, so I accepted that offer. Oh, I didn't have to send my current grades there - just my IELTS certificate (I had an 8.0 for that).

So why do I have a problem? I just had my final exams and there's a 90% chance I am going to fail. Now I have no idea what to do - should I go without getting my high school diploma? Or should I get my diploma before moving to the UK? I can always get my high school diploma after studying.. And how do I convince everyone I should go there without getting my diploma first?