Crossings are Dover Boulogne - which is South West of Calais and has never had traffic problems when I have been there. Its where the old Speed Ferries sailed to prior to falling victim to the recession.
£20 each way car and two passengers - any duration on quieter crossings or far in advance.
£18 for a motorbike and two passengers
Also starting at £28 for a car and four passengers
This port if best if travelling south - if you intend to go east into Europe then Norfolk Line to Dunkirk is best.
The following crossings are also available - but are pretty long crossings and best if you plan to see the north west coast.
Portsmouth - Le Havre £20
Newhaven - Dieppe £20