A note to Americans and a few other nationalities, this refers to The National Archives of Great Britain at Kew, London, not to the National Archives of any other nation. From The National Archives web site at http://bit.ly/2xBGvrh:
With effect from 14 November 2017, large suitcases, bags and other items that are too big to fit into our lockers will not be allowed into our building. This change is being introduced for safety and security reasons, and is in line with arrangements at similar institutions across London.
Our lockers can accommodate items with a maximum size of 43 cm x 42 cm x 24 cm. Please make arrangements in advance of a visit to leave luggage elsewhere.
This change does not include pushchairs or baby buggies, which are still welcome in our building.