When Britain stopped expecting invasion

When did fears of a German invasion of Britain recede?

They diminished after Germany invaded the Soviet Union in 1941 and had committed the forces needed for a cross-Channel invasion elsewhere, with the shift clear by about 1942.

The answer is strategic rather than tied to a single cancellation date. Once Germany committed major forces eastward, sustaining an invasion of Britain became far less plausible. Defensive preparations remained, but the urgency behind stop lines and beach defenses declined.