Look at the volatility of the pound. It's not moving with news about the long term effects of Brexit, it's moving with the short-term political news. The range

Look at the volatility of the pound. It's not moving with news about the long term effects of Brexit, it's moving with the short-term political news. The range of uncertainty is incredibly large: when, how, and whether Brexit happens is still up in the air. Conditioned on Brexit actually happening, a prediction of a weaker pound is reasonable.