I think the problem may be your remaining subjects since some unis will consider Maths and FM to be too similar to be counted as 2 separate subjects, unless you are applying for Maths courses. Of course you also applied to very competitive unis so that won't have helped. It's likely that you would have got offers for Maths, but not for CS.
If you don't get an offer from Kings, you will be able to make other choices (one at a time) through UCAS Extra so you don't have to wait for Clearing. But contact the unis first to check that they are happy with your subject combination.