Britain’s top 25 companies are joining together on Thursday to launch a greenhouse gas emissions-trading system in an attempt to persuade the government to drop plans for a carbon tax that could cost polluters as much as $3.2 billion a year. A spokesperson for BP Amoco said the companies, which also include Royal Dutch Shell and British Airways, hope to have a prototype of a national trading system in place within a year to help the U.K. meet its targets to reduce carbon-dioxide emissions under the Kyoto Protocol.