You should inform Cambridge when you make your application that you can't take HL maths 'cause no one at your school is enabled to teach them. They take that sort of thing into account. In any case, if I were you I'd try to teach myself HL maths and take the examination anyway if you can, it will certainly help you a lot both in your application and for your interviews.
You mention that your maths skills are at a high level, haven't you taken any examinations that prove this? I think they could probably be taken into account by Cambridge, provided they're official examinations... You might also consider checking out what Maths exams you can take without being taught officially... I hope this helps. Good luck!