Have a look at P&O's website. ( www.poferries.com ) I have just booked a special deal where I am going just for the day for a tenner with the car, but there are plenty of other offers. Just had a quick look, but basically anything from £70 for a week upwards!
I know the Chunnel is great, fast, easy etc etc but you can't beat a little wander around on deck, and for the sake of the extra 40 or so mins it takes you, I think its worth it.
cheapest standard fares with car are with Seafrance. To get the cheapest fares go for the less popular times of day like early mornings before around 9.00am, and coming back late afternoon early evening or overnight. Its well worth playing around with ferry times on the websites as there are quite substantial savings to be had travelling on the less popular times.
Just got great deal of £90 return for 9 days end of June . Offer from P and O.
Call 08705 202020 and mention the newships offer as advertised on radio and bingo cheapest deal around. Only stipulation is that travel must be prior to mid July. It is £10 a day that you are away for I think.
I've recently taken to using the Dover-Calais Hoverspeed Seacat for my business trips to the Continent. Almost as fast as the Chunnel and certainly much cheaper. It also has its own dedicated terminal (the old Hoverport) so loading/unloading all seems very civilised. It can be a bit lively in a blow but that shouldn't bother us yotties!! Now that I'm on their mailing list I get advised of special offers. Currently they are offering £59 each way for a car +5 up to the end of June. Check out their website on www.hoverspeed.com but make sure you look up the special offers first and then input the appropriate code when requesting a quote. Otherwise you may just get the standard fare.
Also try www.ferrysavers.com. As others have said you have to work hard with the internet, juggling dates times etc. to get the best deals.
P.S. Just flown to Pisa (from B'ham) and back for a £1 each way (+ tax £50 for two). Best fights I've had for a long time. The only niggle - a week after I booked the price fell to £0!