Your US videos beat your UK videos. Was it the country?

You changed market and creative together. The result may identify a promising combination, but it does not isolate which change caused the difference. Preserve the observation without giving it a stronger explanation than it supports.

For the next test, choose the decision you need. Compare a similar creative idea across appropriate market versions, or compare two creative formats within one market. Keep post age, destination and success definition visible. Operational tests rarely control everything; document the remaining differences.