Hey there!
I just firmed my Unconditional offer for ESPS at UCL. Here are a few of my thoughts on your post (although I say all of this will little legitimacy as I don't even live in the UK, but they're just intended to help you think this one through).
1. I don't know how easy it actually is to change courses once you get to UCL... But since ESPS is (from what I have heard) one of the courses with the best reputation at UCL (especially at the Arts&Humanities Faculty) you might have a better chance of going to Phil&Econ than the other way round.
2. That brings me to the prestige of ESPS. Although I never really did research on Phil&Econ, I heard of ESPS without doing research really (through gossip about good political science courses; note that I don't live even close to Europe). Moreover ESPS is a unique course in what it offers (a 'genuine' year abroad at a local university studying in the language of the country) with a great deal of interdisciplinary (compared to other UK courses). These are unique features of the degree which rightfully give it a good reputation, and a solid added-value which you put forward to potential employers.
3. Honestly, German (the language I am going for as part of ESPS) is not the language I am most interested in (which would be Persian) but it fits very well in my current interests and it will certainly be invaluable knowledge that very few other British graduates will have (at least the depth of knowledge ESPS guarantees through its unique study abroad). And I plan on taking courses in Persian outside UCL, maybe not in the first year not to get my languages mixed up, but that's far from your problem. I guess what I'm saying is that if you're already committed to a language I think you can trust yourself to keep it up, and then use your holidays to go on language programmes in the PRC. That's what I intend to do with Spanish at least (which I have also been studying for the past 8 years or so).
4. The fact that ESPS is four years instead of three, I think, can only be seen in a good light. When I applied to my universities, UCL was at the bottom of my list precisely because it was a four year course. Then I spoke to a few friends who've graduated (with MAs and the like) and they said that a four-year course with such a 'real' study abroad experience will give you many more contacts than you might accumulate during three years, in London only. And in the end it is through these contacts that you will be able to find interesting job opportunities which fit well with your qualification. Some of my friends even suggested that a four-year BA like ESPS (which also includes a Dissertation component uncommon to many other BA courses) might provide you with enough opportunities and time to grow (in maturity, confidence with the subjects your dealing with, in social skills, in self-confidence, in academic rigour, in your extra-curricular activities, in your holiday work/internships) that you might not have to immediately go into a Master's following your BA. Instead you will probably have more interesting opportunities to work immediately, albeit not as a managing director or anything like that.
5. Finally, about going into Investment Banks/Consulting Firms: hasn't this financial crisis taught you anything?! Hahaha, no. I'll keep that argument for when we meet if you come to study ESPS. On a more serious note, I think a lot of what will differentiate you from other applicants to jobs in such companies will depend on: 1. The grades you get on whatever BA you study 2. The recommendation letters your professors will write (thus the impression you make on them during your studies) 3. How you used your holiday time (chilling on the beach, learning/perfecting a language, enrolling on summer programmes, finding internships, etc.) 4. The languages you speak (and any other concrete skills you might bring to the company). All this said, I will agree with you that Phil&Econ will look more appropriate for such jobs in terms of the knowledge it teaches you. However, as you pointed out ESPS will give you this knowledge, and more: another language (again, I'm not worried about your Mandarin, honestly), a broader perspective (European, not British), a proven ability to go out of your comfort zone from a relatively young age (few are those who'll willingly enrol on a course knowing that they'll have to spend a year abroad in a country whose language they don't speak yet, or a course which has a mandatory 10,000 Dissertation). I think once you get to the interview stage ESPS will be as strong a course to sell as Phil&Econ, if not stronger.
I hope this gets you thinking. You can obviously see I am biased towards ESPS, but I think that's because I've wholly accepted that this course has unique features which very few other courses offer (a simple exercise: count the number of universities that offer Phil&Econ BAs and then count the number of unis that offer BAs similar to ESPS, then think about which course is going to make you a different/unique/interesting candidate for a job).
Good luck! See you in September hopefully