KCL is patchily good, but where it is good it is very good indeed, and International Relations is a particular strength. I see this course as more appreciably more prestigious than Law at Warwick, never mind that 'with social sciences'.
With regard to social life, Warwick is a campus university, which confers a set of benefits for making friends. But after living on site for year 1 the Warwick students spread more widely than is true at any comparable school: they don't spread themselves between different towns at Bath or York. And social life doesn't consist only in being able to drum-up a group to go down the Leamington Spoons, central London is an education of itself. Dreary Sunday afternoon? Pop into the British Museum.
I'd go for KCL every time here, with only the difference in costs perhaps prompting pause for thought in it.