I believe voyages to Britain took about a month at most usually, or two months if you faced bad weather and the wrong wind direction constantly. I'm not sure if the Greeks or Phoenicians would create colonies - I really don't know enough about their mindset to judge - but I'll point out that the Greeks did trade with Britain already. Indeed, one Greek (I forget who) actually circumnavigated the British Isles and drew a map of them. I think Polybius had a fairly extensive (if inaccurate) book about the various tribes and customs of Britain, too...