Why did opel change their name to vauxhall in the UK?

Opel were never present as a company in the UK, so never made a name change. Opel in Germany and Vauxhall in the UK were separate well-established companies when they taken over by General Motors in the 1920s. They continued as more or less independent entities until the 1980s when their activities and models were merged, but the names continued in their respective countries.