Why Britain admitted Japanese fact-finding missions

Why were Japanese visitors allowed to study British fighters in 1940-41?

Britain still sought peace and hoped a controlled display of capability would deter Japan, while normal diplomacy made outright refusal difficult.

Britain still hoped to avoid war with Japan when Japanese visitors came to learn about Spitfires and Hurricanes in 1940-41. Showing modern aircraft and military preparedness could help deter a prospective opponent. The visitors also belonged to continuing diplomatic missions and relations. Refusing access would therefore have created diplomatic difficulties at a time when Britain still sought peace.