Even within British English, the forms 'ascian' and 'acsian' existed side by side at one time. The latter gave rise to 'ax', which was commonly used until the 1600s and thereafter used in various British dialects as the normal form of 'ask'. I'm not certain, but I wonder if it is possible that British explorers, missionaries etc who first went to Africa actually took that form there with them. It would, after slavery, have been natural for it to be common in the West Indies, too.