The Audi Quattro used by Gene Hunt was not available for sale in the UK in 1981. He would have had to order it from continental Europe, thus it would probably be left-hand drive. This is acknowledged to in the extras, which speculate that Hunt could have gotten the car as "a dodgy import."
There are many cars out of place during the course of the series, including a 90's era Peugeot 205, and late model Mercedes. Many of the stock cars are repeated throughout the show including a red Vauxhall Cavalier.