Mr Red.
You need to look at more than just the headlines. The 'slump' in sales is only by comparison with the two previous years 2015 & 2016, which were record years for new car sales in this century.
Sales in 2017 were lower than in 2015 & 2016, but on a par with 2002, 2003 & 2004. They were higher than any other year this century.
Hardly a slump, just 2015 & 2016 were 'bucking the trend'.

Radio 2 news this morning (not usually a supporter of Brexit) made it clear that the 'fall' in sales was noticeably greater for Diesel cars, thus suggesting that uncertainty over these may be an important factor in the reduction.