Basically words like environment and sustainable development have very broad and wide ranging definitions, so as you've identified you really need to knuckle down and focus on what specific areas you're interested in.
also try
environmentjob.co.uk and http://www.lowcarbon.com/ for jobs and voluntary placements.
Voluntary work might be a good bet for the time being. Get some proper experience under your belt and decide what you'd like to be doing.
By the way there are quite a few jobs available in wales at the moment as the welsh government insists on implementing it's own versions of environmental policies and regulations, which leads to jobs not only in government but in ngos and consultancies as well.