P&O Ferries are offering day trips from Docer to Calais from just £19 return for a car + 9 - and a free bottle of wine.
As long as you can get down to Dover early enough (bearing in mind the fact that France is 1 hour ahead of UK time) then you could have a leisurely lunch and do a bit of window and/or hypermarket shopping before an early evening sailing back to Dover. And Boulogne, Arras and Dunkerque are all within less than an hour of the ferry terminal.
Effective French fuel prices have risen slightly since January 07 but are still quite a bit cheaper than the UK - especially for diesel. With a tourist exchange rate of £1=€1.41 the following prices are likely to be found at the more competitive outlets (i.e. not autoroute service areas)
Unleaded (sans plomb 95) €1.179 = 83.4p/litre
Super unleaded (sans plomb 98) €1.229 = 86.9p/litre
Diesel (gazole) €0.998 =70.6p/litre
