Marketplace: Renita Jablonski: One of Britain’s leading energy companies has warned wind power is becoming unprofitable in the U.K. From London, Stephen Beard reports. Stephen Beard: Centrica says the economics of offshore wind farms are now marginal. The cost of construction doubled in the past two years. The soaring price of steel and copper are mainly to blame. This is the second attack on the viability of wind power here in recent days. Last week, the Shell Oil company delivered a devastating …