I've done a couple of the non-invasive, psychological type tests. I did one to earn a few bucks. Later, a local University was doing a study on anxiety disorders. I have an acquaintance who suffers terribly from anxiety, so in this case, I was happy to help.
When I lived in DC, there were clinical trials ongoing all the time, advertised in the newspaper. They did (do) pay fairly well, but the majority are things of which I would not want to be a part (testing malaria vaccines, for example).
I'm not sure where you're located in the UK, but try checking with the local universities perhaps, for some less "scary" research participant opportunities. I see stuff posted all the time around Oxford for various research projects.