Not sure that's the case - many drivers on the ferries have tanked up in France rather than do it on this side of the Channel (well, they certainly used to)?
That's because I wasn't sure - I know it used to be the case. People would nip over to Cité Europe to buy their washing powder and cold meats, and fill up while they were there.
What I use for petrol price comparison in the UK is PetrolPrices.com app http://www.petrolprices.com/ It would be great to have a similar tool for comparing European fuel prices...