if you are a UK national and looking for a career in conservation/ecology then volunteering in the UK will be much more useful to you in the long term. it will also be cheaper. the rspb has residential volunteering at a number of reserves. depending on your interests/ability there may also be a number of surveys you could get involved in. you can then spend your money on extra training or a nice holiday.
most overseas programmes are for profit and part of what you pay goes to that. if you want to experience looking at charismatic megafauna somewhere exotic then fill your boots but, depending on your objectives, it is unlikely to be the best use of your time and budget.