True ignorance of plagiarism while creating a piece of writing is, to a point, excusable. Putting it out there without checking your work, however, is not. With current detection tools, nearly all mistakes of this nature are extremely easy to catch, and increasingly, there's no justification for failing to do so.
The broad area of intellectual property, protecting people's rights and copyright law in particular is hugely complex to most people. Centuries of tradition and common law lie behind what is in place today in the UK with the geographically-based rules and regulations -- hardly a good fit when much content today is digital, and certainly not based on physical geography.