Hey VLC,
I took ESPS and I was very satisfied with what I got. Of course it really depends what you're looking for. What you should know is that ESPS is a very broad degree that allows you to do pretty much anything you want. That can be an advantage, if, like me, you want to discover new academic fields. On the other hand it can also be a disadvantage if you know exactly what you want to do.
In my case, I did a specialisation in law, which was enough to make me realise that's what I want to do. A few years down the line, however, and I'm having to start a law degree from scratch because whatever modules in law I did in ESPS are not enough to qualify as a practicing lawyer - so I'm regretting not studying pure law from the start. But as I said, when I started at UCL I didn't really know what I wanted to do, so I'm thankful to have had the opportunity to try loads of different subjects, otherwise I would probably never have realised that I like law so much in the first place.
In your case, you seem to already know which direction you want to go. If you want to be hired by McKinsey/KPMG/Ernst & Young, you really should go for a more business-oriented degree. Economics, or MBA, or even Law & Economics (v. good degree by the way).
The appeal of ESPS is in the language + year abroad, but you can actually take a year abroad with most degrees nowadays, so if you organise your economics degree in such a way as you would go abroad for a year and learn a language, then you don't necessarily have to miss out on that.
That being said if you do take ESPS, I'm sure you will have a very enjoyable four years and you will learn a whole lot of very useful stuff. The professors are all top-class, and I consider myself lucky to have done the degree. Also, the department is small enough that it gives a family-like atmosphere. Not something you get in the bigger departments.
The only thing is that the job market is getting really tough out there and it just seems like employers are moving away from arts degrees altogether, so in terms of strict employability, a BSc in Economics or dual LLB/Bsc is definitely a better bet.