What do you mean by "ancient". If you're thinking of ships in the time of Homer's Iliad and Odyssey, that is about 800 BC, it is doubtful if they would have been able to complete such a long, hazardous journey; and in practice it would probably have taken many months, if not years. If you're interested, there is an excellent website on ancient ships at http://www.artsales.com/ARTistory/Ancient_Ships/09_epic_poem s.html
There is plenty of evidence the Phoenicians traded with Cornwall at least 3000 years ago and some almost certainly settled. If so why not the Greeks? I've no idea how long it would take http://www.nationmaster.com/encyclopedia/Phoenicia