What about somewhere within Eastern Europe such as Romania, or even Turkey? They are both strikingly different cultures to the UK, especially once you get out of the cities and into the countryside. Turkey is really interesting as a Muslim country too. I'd say Russia too, but the visa problems for there might be too difficult as I don't know how you'd go about getting a volunteer visa.
I totally understand what you're saying, as Nepal is a lot more exotic and exciting than somewhere in Europe (and I don't know where you've been in Europe) but once you get into the poorer parts of this continent, and into the backs of beyond so to speak, the cultural differences and ways of life are obvious. You don't need to go far to experience that. Even within the UK there is a completely different way of life in London to on a tiny island community in the Outer Hebridies, for example.
As you've said, you need to warrant this year off to universities when doing your UCAS. However, I don't think they would really care if you'd spent 2 weeks volunteering in Nepal or 2 weeks volunteering elsewhere - volunteer work is volunteer work.
Although I haven't been to Nepal, I have been to that region of the world (I was in Mongolia then China for 4 months this summer) and trust me, it's beautiful. You owe yourself more than 2 weeks there. I would hate to see someone go all that way then only touch the surface of such a facinating cultural world (Mongolians 'share' with Nepal in terms of religion, culture, ideology etc) these countries are so surreal and so different that you have to get used to being there, and let yourself be absorbed by the people and communities you are met with. This takes months, and two weeks wouldn't do the area any justice and I feel you'd leave simply regretting that you can't experience more of it. Obviously you always think that when travelling, and you always say 'well I'll come back so its okay' but I certainly think that travel across the globe shouldn't be entered into lightly, to get anything out of it you have to commit for a longer period. Otherwise you may as well not bother going, as you won't really learn anything in 2 weeks. I'm going back to Mongolia next summer as I've fallen in love with the country, I haven't seen enough even after serval months of living and working there. I speak basic Mongolian too, which is also a thing you'd miss out on only being there for 2 weeks. I think you'd be much wiser to go in a years time after first year of university, then you've got around 4/5 months to play with so you can actually make a difference volunteering somewhere, as well as making a difference to your own life and perspective.
Sorry... I'm an anthropology student - extended exposure/experience to other cultures is my academic life as well as deep in my travelling obsession! The two go hand in hand in my life.