For us it is most convenient to drive to Calais (France) and take the ferry from there. In all occasions we booked at P&O Ferries, but SeaFrance is also an option. Both go to Dover (Great Britain), which suits best for our journeys.
If you would like to visit Great Britain by car, you can either cross the Channel by ferry or by train (Channel Tunnel). We have done both, but because we like to travel by car we prefer the crossing by ferry.