I understand that officially the BC runs east of a line from St Govans to Hartland (though other definitions are bit further east), and its eastern boundary is Lavernock to Sand Point.
I think barebone's idea of sea area Lundy to be a wide enough area /forums/images/graemlins/smile.gif
I have another definition for the Severn Estuary (which I've just thought of!) - as soon as you start catching mackerel in the summer, you must have reached the Bristol Channel ie the characteristic brown gloop has gone. This would make the dividing line roughly Foreland to Swansea Bay.
And if you think about it (I just did!) then if it is clear enough, if you can see both coasts from the deck of a ship you must still be in the BC (though I don't fancy climbing my mast to check out the theory /forums/images/graemlins/frown.gif )