Did Britain cease to be a first-rank power in 1945?

Did Britain stop being a first-rank power immediately after the Second World War?

No. It retained global strategic reach after 1945, though the transition from empire to Commonwealth gradually reduced that position.

Britain did not move from first-rank to secondary status on a single day in 1945. Its navy, imperial network, and worldwide presence still gave it strategic reach across many regions. Early Cold War incidents involving Albania could still be handled without relying on the United States. That position weakened through the transition from empire to Commonwealth rather than disappearing on one date.