If you are travelling by car from the UK to Portugal, you have 2 main options, via France or via Spain. Here is information about both routes.
There are many option for crossing the channel, the prices for the ferry are much cheaper but it is a longer drive to Portugal, so you do need to consider the cost of fuels, tolls (expensive in France), extra meals and even stop-over accomodation.
By the time you have added everything together you normaly find that the Spanish routes are more economical and faster. However if you are travelling in the winter and you are prone to get seasick then the French route might be the best for you. Winter ferries are often delayed or cancelled in very bad weather.
For a quick calculation of fuel and tolls costs go to www.viamichelin.com
Is is useful to use a ferry booking agent like Direct Ferries, so you can see all the options and prices, rather than searching through the individual ferry operators websites.
For England - France journey there are 14 different routes, the operators are P&O, LD Lines, Brittany Ferries, Condor Ferries, Norfolkline,Seafrance and Transmanche Ferries. There is also the Eurotunnel option.
Irish Ferries have routes from Rosslare, Ireland to France (3 locations).
There are only 3 options from the UK to Spain and all routes are operated by Brittany Ferries.
There are 2 crossing per week between Portsmouth and Santander on their flagship MV Pont Avon or the new MV Cap Finisére. This crossing takes 23 hours and 30 minutes.
The route from Portsmouth to Bilbao, has two crossing each week on MV Cap Finistere. There are two routes taking either 24 or 32 hours.
All winter crossings are subject to cancellation or delay due to weather, and you need to have good 'sea legs' from crossing The Bay of Biscay in a storm.
The route from Plymouth to Santandare operates weekly from mid March to early November on MV Pont Avon and takes 20 hours. This is the shortest of the UK - Spain crossings.
From Santander or Bilbao to central Portugal (eg Tomar) will take 8-9 hours of driving, and 2-3 more to the Algarve.