The FT has the answer:
Of course, as many will point out, a decline in the currency is good for exporters. It is also good for the foreign currency earnings of the multinationals listed in the FTSE 100. But unless a currency is overvalued (as the pound was in 1992), it is folly for a nation to celebrate a sharp decline. If devaluation were the answer to economic success, people in Venezuela and Zimbabwe would be rich. A weaker currency is a decline in the terms of trade; it costs more for citizens to buy foreign imports they want and their exports are lower in price. As a trading nation with a current-account deficit, Britain is dependent on the kindness of strangers; the willingness of foreign investors to send capital.