Crown Copyrighted Works: License Information

Authors of this article are employees of the British or British Commonwealth Government and prepared the article in connection with their official duties. As such, the article is subject to Crown Copyright and is not assigned to IEEE. IEEE has been granted the right to publish, distribute and reprint the article in all forms and media. Guidelines for reuse and distribution of Crown copyrighted articles can be found online at https://www.nationalarchives.gov.uk/information-management/re-using-public-sector-information/uk-government-licensing-framework/crown-copyright/