Task Brazil accept people with fluent Spanish or a good level of communication in Potugese. Sponsorship costs are between �1200 and �2500 (but these figures do include flights):
http://www.taskbrasil.org.uk/
Depending upon your religion (or lack of it), you will have your own thoughts on working in an orphanage which specifically aims to raise children as Christians. If this ethos ties in with your beliefs there is an extensive, worldwide list of Christian orphanages here:
http://www.missionfinder.org/orphanages.htm
(The first half page is concerned with US orphanages. Scroll half-way down for the start of the international list - this continues over several pages. Not all of the orphanages are seeking UK volunteers - it will probably only be a small minority - but there's a great deal of reading to do and you'll probably find something of interest).
The Global Volunteer Network is based in New Zealand but they accept UK volunteers for their work in Romania. The costs seem quite reasonable:
http://www.volunteer.org.nz/romania/
Here's the GVN link for orphanages in Nepal & Vietnam (but these will probably require considerably more funding):
http://www.volunteer.org.nz/nepal/
http://www.volunteer.org.nz/vietnam/orphanage/
Hoping something here is of use,
Chris