Roosevelt regarded this offer of a joint project as sufficiently important to have the letter personally delivered by Frederick L. Hovde, the head of the NDRC mission in London, but Churchill did not respond until December. He assured Roosevelt of his willingness to collaborate, and informed him that Hovde had discussed the matter with Sir John Anderson and Lord Cherwell, as Frederick Lindemann was now known. The MAUD Committee had considered the issue of collaboration with the United States, and had concluded that while pilot isotope separation plants could be established in the United Kingdom, full-scale production facilities would have to be built in the United States. The British expressed concerns about the security of the American project. Ironically, it was the British project that had already been penetrated by atomic spies. John Cairncross had given the Soviet Union a copy of the MAUD Committee report. Although not conveyed to the Americans, the British had other concerns about what might happen after the war if the Americans embraced isolationism, as had occurred after the First World War, and Britain had to fight the Soviet Union alone. The opportunity for a joint project was therefore missed. British and American exchange of information continued but their programmes remained separate.