Maybe they are pushing it to the destination through the tunnel under the channel when the trains are not driving to decrease the shipping costs
Really it is hard to explain why do they need so much time... It is more expensive to ship Clio Sports from Dieppe as it is their only production model which is sold in just couple of thousands per year than normal Clios, but I would say that all those additional costs are built in the price. RS is like 30% more expensive than the GT which comes with more standard equippment, at least in Croatia