Is it okay to add 15-20% of your own data to a copyright document and then make copies to pass to people?
The short answer is no.
The long answer is this:
If a document is protected by copyright, then under the Copyright Designs and Patents Act 1988 the copying of a "substantial part" of that document without the permission of the copyright owner will constitute copyright infringement. It is immaterial whether additional material has been added to the document - whether a derivative work has been created.
Even if you have added so much material that only a small fraction of the derivative work comes from the original document, then copying will still infringe the copyright. In other words, the question is whether a substantial part of the original work is copied, not whether the original work forms a substantial part of the derivative work.
In this context, I should also mention that substantiality is assessed in a qualitative as well as a quantative manner, so even a relatively small part of a work (e.g. a few lines from a poem) may be substantial.