the disparity in this US vs UK is silly. It does sound like it's overly hard to get melatonin in the UK, but I think it's probably too accessible here (every grocery store). It's definitely a psychoactive drug, and given that it's a hormone which is intimately connected to the pathways that produce serotonin, n,n-dimethyltryptamine and other fundamental molecules of human consciousness, I think people should be a little wary about just gobbling tons of it all the time.
That said, I once spent a weekend taking pretty obscene amounts of it to see what would happen, and I don't think anything bad happened to me as a result. That experience definitely sold me on "melatonin is a drug", though. If you don't believe me, go take about 500mg over a 72 hour period...