Salut, FB!
I don't know, whether it helps, but I just found a very interesting website, which was www.bargione.co.uk, found via "volunteering London NGO" at Google. A lot of recruitment agencies are listed there, which deal only with volunteering jobs or ..., I don't think you would mind to get some money..., with temporary jobs in non-governemental organisation. Try there.
Hope, this helps!
Birgitt