I don't think boundary changes are the answer. Cumberland, now Cumbria, includes the English Lake District and is most easily thought of as that large bumpy bit of coastline directly opposite the IOM and just south of Scotland. Directly below that is Morecambe Bay, and then you have the whole of Lancashire and a bit of Cheshire before you get to North Wales. Borth/Aberystwyth are mid-Wales coast, 'new' county of Dyfed, so definitely not bordering Cumberland. I expect someone who lives in the area could be more accurate in their description than me, and perhaps I should have consulted an on-line map before I started!