(Reuters) - British car dealership Lookers warned on full-year profit on Friday, saying trading was increasingly challenging and prompting a 30% fall it its already weak shares.The British car industry has been hit by lower vehicle sales amid uncertainty caused by stricter emissions regulations, Britain's impending exit from the European Union and a shift towards sales of electric or hybrid cars."The more recent challenging conditions are likely to continue into H2, exacerbated by continued..