A high-level British offer of help to clean up the Gulf of Mexico oil spill was rebuffed by America shortly after the accident, fuelling fresh fears of political tension between the two countries over the disaster. A few days after the BP-leased rig sank on April 22, the Cabinet Office made a direct offer to the US State Department to airlift half of Britain’s 1,200-tonne stockpile of chemical dispersants, The Times has learnt.