I'm less accents, and more into studying languages over all. You'd be surprised how many similarities there are between Old English and Latin, despite them not actually being related languages.

So, as for "yeah" vs. "yea", I'm mostly going by what I hear, in the form of the few British people here (America) I know. That yah/yeh thing is pretty much the difference I've noticed. (Then again, Valley-girls tend to pronounce "yeah" like "yaaah", so who knows, maybe my ability to hear that word is botched up from constant exposure to Vally Girl stupidity or something).