Organise it yourself/with some friends or students - save up for a bit longer, head over to Cambodia yourself, liaise with a school there (whilst avoiding landmines,), plan out whatever works need doing and then mobilise the English volunteers and fundraisers necessary. Let the locals do the labour - you have more to contribute than picking up wood and mixing cement.
That way, you are certain that all the money can go to the local communities rather than helping to pay the £30k a year salaries of the charity workers. In terms of what you gain, you can put on your CV that you did all of the above by yourself, showing real independence, organisational skills and bravery.
Let's be honest, it is 2012 and this kind of package tour volunteering is not a new idea - a more cynical recruiter might look at this and think "hmm, so he paid 2 grand to ineptly lay bricks and flail around as an 'I speak it so I can teach it' English teacher for three weeks?" Not quite worth the £2,000 when its put like that.