As you know, In OTL, The son of Alexander the Great upon the end of the Third Diadochi War in 311 BCE, the peace treaty between the Generals acknowleged the young prince Alexander IV to succeed Cassander when he came of Age...
Unfortunatley, things didn't work out as planned and champions of the Agread Dynasty demanded that he should hold the thrown, ended up getting him killed at the age of 13...
But What If...Those same supporters of Alexander IV right to the Macedonian Throne knew very well that Cassander had no intention of relinquishing his power and decided to get as far away from the Diadochi as possible. They decide to move the Prince and his mother to the Isles of Pretani around 310 BCE a land explored recently by the great PYtheas. By either over land routes or by sneaking through the Pillars of Hercules they arrive at Cornwall, and settle down to make a significant Greek Stronghold on the Island.
So how would a major prescene of Greeks affect a Celtic Britain at this time? Would Carthage most likley attempt to smother out the colony before they have a chance to expand? How many Greeks would end up travelling to the Isles knowing that the Bloodline of Alexander holds strong there? Would Cassander follow them or would he be to busy consolidating his power to worry about them?