The best approach here is to use the official gCO2/km figure for your car: go to the Car Fuel Data website and locate your exact model. Type the emissions figure into the box and choose "gCO2 / KM" from the pop-up menu.
If your car isn't listed there, try Parker's instead: locate your model and click "Technical data" on the right. Type the MPG figure into the box and choose "MPG" from the pop-up menu.
Don't forget to specify petrol or diesel.
DEFRA: petrol 2.30kg/ltr, diesel 2.65kg/ltr
There are major differences between the various emissions figures for air travel. The DEFRA figures seem to ignore the "radiative forcing" effects of CO2 and water vapour emitted at high altitude that make air travel so damaging. For a discussion of this, see here.
Given that some of us have resolved never again to choose a holiday that involves flying, the figure for the ferry alternative is depressingly high. Maybe it assumes that all passengers are travelling with a car? For a discussion and sources, see here.