My 2c:
Reputation-wise, in general terms, UCL has a stronger reputation globally, Warwick is on par in the UK and maybe slightly ahead in Asia. If you spend an Erasmus year at a leading continental European university (including Sciences Po) and/or are going to apply for big organisations or companies there, it shouldn't matter. For work in the US, both would be a bad choice.
Ask yourself what kind of student lifestyle you want. I've lived both in London and right next to the Warwick campus. While London offers many opportunities, I didn't like the hectic too much. But, then again, I got a little bored at Warwick after half a year (but I've been a post-graduate and most union activities are heavily geared towards undergrads so this could be completely different for you. Most undergraduates I've been talking to enjoyed their time there).
Most importantly, however, ask yourself where you stand ideology-wise. At UCL, you will only be taught in one economic perspective: hardcore neo-classic. Economics degrees at both universities are quite quantitative, but there are at least a couple of economics professors who're open to alternative views at Warwick as well. PaIS is quite diverse - and, compared to UCL Econ and SPP, clearly more centric/left-wing. I'm generalising and it always depends on the lecturers you might have, but if you tend to agree more with <insert any centrist- to left-wing ideology here> than <right-wing ideology>, you might not be too happy at UCL. I have no idea how economics is taught at Sciences Po, but I suppose they will have no extreme neo-classical focus which means that you might even be irritated when going to UCL afterwards as you don't get what they want from you.
Last but not least: despite taking a couple of philosophy courses as a PPE student, don't expect to get a real clue about philosophy if you're not really interested in and proactive about it. It's merely an introduction which is being offered (and some would argue not even that but just a 101 in ethics and stuff). Still, I like the PPE and EPaIS combinations at Warwick (I had an offer a few years back).