Wim,
I guess I got confused here. The US National Archives is still closed. The National Archives/PRO in Kew has reopened to a certain extent. They have just announced further reopening steps, see
https://www.nationalarchives.gov.uk/...avirus-update/
Since Covid-19 travel is probably not feasible (quarantine?) you will probably have to wait until research-contractors are willing to return.
Enjoy!
Frank.