Drive the car to Eurostar, drive on a train, get off in Paris and drive to site.
Just make sure you comply with the foreign vehicle regulations whilst in France or they will fine you and impound your vehicle
Do you mean drive to Ashford, get on train and get off in Calais? The Eurostar service is only for passengers, not cars.
TBH the Dover Ferries are so much cheaper than the eurotunnel, one car between four people I am looking at 25quid each including crossings and autoroute tolls (excluding fuel). Whereas, the last time I looked at the train it was 147quid.
As for French motoring, stay on the right, its a myth that they are queuing up outside Calais and on the autoroute toll booths to nick English drivers. There is a list of reauirements on the AA/RAC websites, most of which is common sense stuff. They will impound your vehicle if its not properly insured, or you have no proof of your entitlement to use it (Log Book/copy and/or letter of consent from the registered keeper).
Hope that helps