Forget about UMS average. That's not what Cambridge uses in their selection process, nor have they 'threshold'. Please be careful about lots of heresays about Cambridge admission you get from internet and other sources, sometimes even your school.
The big question, still, is if there a particular reason why you are not doing as well as you hoped in AS? Was it just because of exam nerves, for example, or any extenuating circumstances that hindered your exam performance? Or have you found AS Geography itself quite challenging thus far.
They look at all the available info about a candidate to see if they will be able to cope with their demanding course. So what they're looking for is the candidates' potential, and the past records is one element of info they use to estimate it.
They need to be assured that a candidate is capable of achieveing A-level studies at satisfactory standard as what you'll be doing at university level (at any top uni, including Cambridge) is much, much more demanding than what you're doing at A-levels which makes A-level looks like a tiny weeny piece of cake.
I hope your exam results will be better than what you're anticipationg, but in case they're not, there's always a possibility of applying post A-levels with solid results.
Also, it's very important to remember Cambridge is NOT the only good uni to read Geography. I happen to know several people who read Geography at Cambridge and also at a few other unis. They're all different in many ways which does not mean one uni is better than others in all aspects, esp. because geography is very broad and fast-evolving subject. Please look through all the details of Geography courses at many universities very carefully. You may find other unis offer a course that's actually better for you.
Good luck with the results.
Btw, no need to start multiple threads with a same question.