An American Investor Holds a British Bond Portfolio Worth £100

An American investor holds a British bond portfolio worth £100 million. The portfolio has a duration of 7. She fears a temporary depreciation of the pound but wishes to retain the bonds. To cover this risk, she decides to sell pounds forward. She has observed that the British government tends to adopt a "leaning-against-the-wind" policy. When the pound depreciates, British interest rates tend to rise to defend the currency. A regression of "variations in long-term British yields" on "percentage $/£ exchange rate movements" has a slope coefficient of -0.1. In other words, British yields tend to
go up by 10 basis points (0.1%) when the pound depreciates by 1% relative to the dollar.
a. What should be the optimal hedge ratio used by the investor if she wishes to reduce the uncertainty caused by exchange risk? (The investor uses only forward currency contracts to hedge this risk, not bond futures contracts.)
b. Detail the factors that could make this hedge imperfect if the depreciation of the pound materializes.