Kings is great, nothing wrong with the place! It's not possible for 100% of people to be guaranteed a place after a Chemistry degree, so you're always going to be stepping into the unknown. Chemistry and Medicine are really very different from each other, so I suppose that's why I find it hard to understand the dilemma :P
I would pick for the subject and not for the place. UCL and KCL are both good and they're both a really similar environment being in the middle of London. Just one is North (and better*) and one is South. If you want to do Medicine, go for it regardless of place. If you want to do Chemistry, go for it regardless of place.
*it's okay, they pay me to say it so don't take this seriously