Centrica, the UK energy giant, has continued to trade in line with guidance provided in its May interim management statements.The group confirmed that earnings for the first half of 2011 are expected to be lower than in 2010. This is in line with previous guidance but the causes are interesting.The firm says commodity costs are going up while retail consumption (of gas and electricity) is going down, hinting at the weakness of the UK economy. The operating profit of the retail business is expected to be 50% less than at the interim stage in 2010.While Centrica expects to make more money from its "upstream operations" (exploring and drilling for gas) the new UK tax regime on operations in the North Sea is expected to chip away at these gains.At 8:35am (28/6/11), Centrica's shares were up by a fifth of a penny at 323.9p.BS