In simplistic terms, most governments in European have sought to reduce air pollution by concentrating taxation on CO2 output - since CO2 output is directly proportional to fuel consumed, European buyers have focussed on fuel consumption which was always much lower on both turbo- and non-turbo diesels than non-turbo petrol/gasoline, although the gap has narrowed with the modern crop of turbo-petrol/gasoline with direct injection.
Although European fuel has reduced in price, due to the Saudi trade war with the USA, it's still very high due to taxation structures which use a fixed duty tax per litre of fuel plus a sales tax based on price - even if the oil companies gave fuel away for nothing, the UK price couldn't reduce below about 80% of where it is right now, because of the fuel taxation structure.