Complex sentence: It was based on the Opel Kadett E compact car and was built in the United Kingdom at Ellesmere Port by Vauxhall until 1989 when production was transferred to Azambuja, Portugal and built by Opel.
Split into simpler sentences: It was based on the Opel Kadett E compact car. This was when production was transferred to Azambuja, Portugal, and built by Opel.