Images from the collections of The National Archives posted on Flickr may be downloaded and reused without permission in any format for purposes of research, private study or education (non-commercial use) only.
Images from The National Archives that are part of The Commons on Flickr are labelled “no known copyright restrictions?” indicating that The National Archives is unaware of any current copyright restrictions on these images either because they are Crown Copyright and the copyright is waived or the term of copyright has expired. All of the images may be subject to other third party rights, such as rights of privacy. You are responsible for obtaining other such necessary permissions for reuse.
It is self contradictory to say “no known copyright restrictions” and not to allow commercial re-use. Thus: FEEL FREE TO RE-USE AS YOU LIKE. This and nothing other is the sense of Flickr Commons.