To cross the Channel with a vehicle, you have two choices: ferry or train. The ferry costs less and takes longer, the train faster and more expensive. Folkestone is the departure point for the Eurotunnel train to Calais; the most popular ferry crossing is Dover-Calais.
Here's an overview of some of the options:
To France, the choices are Dover–Dunkerque, Newhaven-Dieppe, Portsmouth to Le Havre, Caen, Cherbourg, or St Malo; Poole-Cherbourg, Plymouth-Roscoff.
The ferry route from England to Belgium is Hull-Zeebrugge. For the Netherlands, Hull-Rotterdam, Newcastle-IJmuiden, Harwich-Hoek van Holland.
As for convenience, that depends on where you start your journey and where you're heading.