There two separate aspects to your question. The first concerns copyright, and in many cases (census entries, BMD certificates) it's a special variety of copyright which belongs to the Crown.
Section 29 of the Copyright Designs and Patents Act 1988 provides an exception to copyright for the purpose of private study and research. Since this exactly describes what family history research is all about, you are able to take advantage of this exception, although it should be noted that you are supposed provide a sufficient acknowledgement of the source when you make a copy in this manner. This is hardly an onerous requirement as we always note our sources don't we?
The second issue concerns the terms and conditions of using sites like Ancestry, FindMyPast and FamilySearch etc. They frequently claim copyright in certain content which they provide such as the transcriptions but even then their t&cs allow you to use this data for private research purposes. Their t&cs do not, and cannot, limit your ability to copy the original documents, such as pages from the census, using the section 29 exception (see subsection (4B of section 29).
Over and above those two matters, section 6 of the
RootsChat terms of use provide fairly detailed guidance on the posting of any matter which is subject to copyright - this is intended to protect the site owners from liability in the event of copyright infringement by individual members. In particular the rules require that any use of Crown Copyright material is accompanied by a notice to the effect that that "This information [is] Crown Copyright, from
www.nationalarchives.gov.uk". This rule appears to apply irrespective of the actual government department which has provided the Crown Copyright material, such as the GRO or Ministry of Defence etc.
