Help! London to Barcalona

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ME and 3 friends are going to cycle London to Barcalona over 3 weeks in july. Does anyone know of any good routes or have any advice they can give for our travels?

Make sure you can swim.

ME and 3 friends are going to cycle London to Barcalona over 3 weeks in july. Does anyone know of any good routes or have any advice they can give for our travels?
Are you using a GPS device? and are your biking for a charity or just for fun?