I got in to UCL and still firmed RHUL, though that was for a number of reasons and your situation is probably different. UCL has a great reputation academically. However the gap between the two isn't the same as, say, UCL and London Met.
KCL definitely isn't in the same tier as UCL though. My friend was a fresher there this year and dropped out at the end of the year because he disliked the university so much. It sounds like there's a real attitude of "we're KCL so you should feel priviliged to be here, if you don't like it then tough ****."
Though bear in mind all three universities have pretty much the same student satisfaction rating. Remember that it's human nature for people to always bitch about where they're from, whether it's school, work, where they live or what country they grew up in. The grass is always greener, etc.
Also the UoL isn't going anywhere fast.