All UK documents are subject to "Crown Copyright", however some for classes of document the Crown Copyright is subject to a limited waiver. For example, documents on the website of the Public Records Office" are subject to the following conditions:
"The material featured on this site is subject to Crown copyright protection unless otherwise indicated. The Crown copyright protected material (other than the Royal Arms and departmental or agency logos) may be reproduced free of charge in any format or medium provided it is reproduced accurately and not used in a misleading context. Where any of the Crown copyright items on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
Images on this site may not be reproduced without payment of a fee to the Image Library.
The permission to reproduce Crown protected material does not extend to any material on this site which is identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned."
Documents subject to waivers vary from time to time. The current list may be found the official site here.