2 weeks volunteering abroad is hardly going to provide a truly meaningful service, especially if it's with a company like Real Gap or I to I (who, by the way, are owned by the travel company First Choice). Sure, you may be volunteering in areas that aren't economically developed but this doesn't mean that they are so desperate they need anything they can get. I'm not saying this as an attack of anything, just so you don't go into the voluteering thing with the deluded notion that you're bettering the world as best you can by volunteering abroad for such a short period with, effectively, a holiday company.
Having said this however, if it's first and foremost a personal experience you're out for, I've found most people that go volunteering abroad for whatever lengths of time and with even firmly capitalist companies [generally find it a personally rewarding experience. (However, that's based just on the research I've done). If you want to do this then, I've heard original volunteers do relatively cheap placements (think it's generally £150 a week) to a range of destinations. But, I'd steer clear of Real Gap and companies like it...I can't imagine your placement being relatively sub-standard compared to what else is out there, but you'll be paying for it alot more then it's worth, with a big chunk going to just Real Gap (which is disheartening, given that you'll be servicing a charity)
You can always do some volunteering in Britian itself. Vitalise (I think it's called) from what I remember run activity holidays in britain for disabled people that require resendential volunteers for the week or two that they run. And, I'm sure there's similar, entirely charitable projects going on that you could find if you did your research.