Eurotunnel  A fast and simple train taking around 40 minutes from Folkestone across to Calais-Fréthun. Day trippers can expect to pay around £60 return including a standard car, longer stays and non standard cars (trailers etc) are charged at higher prices.

Having said that, the drawbacks are that the facilities on board are limited and you're sat in your own vehicle for the duration of the crossing.

Ferry   Travel from Dover, across to Calais in France. P&O, LD Lines, Seafrance and Norfolkline are the operators (Norfolkine operating to Dunkerque, a few miles north of Calais). This will be around a 90 minute crossing with full on-board facilities, shopping and catering, including bars. Day trippers and short break customers can expect a deal usually priced within a few pounds of the Eurotunnel option.

Service 'quirks' exist between the operators, and travellers will have to consider their needs in advance of travel.

Seafrance charge a supplement at the port if you wish to return on a different ferry than the one specified on the ticket, whereas P&O generally allow usage of a different ferry within a couple of hours without supplement.  P&O overall have a more frequent service. 

Seafrance also levy a "Marine Conservation" charge which (despite their claims) is not optional.  Both Seafrance and P&O reduce catering facilities on some early and late crossings.

From the north of the UK, there is a service from Hull. This service takes a few hours crossing to France and uses the port of Zeebrugge in Belgium. Whilst facilities on board are excellent, the cost of the crossing could mean it makes more economic sense to drive to Dover or Folkestone.

Another worthwhile option is the now established operator Transeuropa ferries. The service is from Ramsgate (near Dover) direct to Ostend harbour operating three / four times per day. The crossing is almost 4 hours however, they do not operate the fluid pricing policy introduced by the other ferry operators. Transeuropa also appear to be making a significant effort to welcome the car passenger in terms of on board facilities. With this service, you disembark right next to Ostend train station linking to Brussels and have the benefits of not having the hour drive from Calais into Belgium.

The routes are becoming quite competitive and day returns are good value for money. The main operators to search are: P&O, LD Lines, Seafrance, Transeuropa ferries, Eurotunnel, P&O North Sea Ferries & Norfolkline. It is well worth checking each website directly as there can be a significant difference between fares for the same routes and times.