it's 750 miles, plus 2 ferry rides newhaven-dieppe and barcelona-ibiza. assuming walking speed of 4mph this could be achieved - non-stop - in around 200 hours.
Realistically, walking about 10 hours a day, it'll take around 3 weeks.
oops bit of a dolly dimple moment for me- my answer made sense at the time I have no idea why now doh ! :O( Mushroom25 makes a lot more sense than me !