Good decision not to post on the KCL board as you're right, they may already be card-carrying members of the KCL club.
My personal opinion would be to take the study abroad option. Both are good universities, but you're going to get more of an experience abroad. Plus, American employers tend to look favourably on British unis. You'll probably be more developed as a person from your time overseas.
In making my decision, I would meditate on the differences between the US and the UK higher education system and try to balance them in your mind.
We specialise a lot earlier here in the UK. We don't do the whole major/minor thing unless you choose a joint honours degree which is slightly different anyway. I think you can pitch this in a way that looks really good to employers. (Although, I would also be happy to argue the reverse if studying at a US uni)
Our classes tend to be a less flexible than American timetables. We also tend to have more independent study time. I'm not 100% sure of US term dates but in the UK they tend to be around 9 weeks long.
London is expensive as hell, though.
Best of luck with your decision.
SS