The boundaries we have for the UK at sea are horrenedously inaccurate. There are an awful lot of lines that are seemingly put in there to give closed polygons (IMO on par with tagging for the renderer - distorting the data for the convenience of some tool). Believe it or not, it's perfectly acceptable to leave them open if we don't know where one side actually is.

Take a look in the Bristol Channel, and you'll find a border - that part is in reality territorial waters. Worse still, look between the Scottish Highlands and the Outer Hebrides and you'll find a border running through what are actually internal waters, since the baseline is drawn around it.

The actual demarcation points for the baseline, including the boundaries between internal zones attributable to the four nations, are defined in legislation, where they are simply listed as lat/lon co-ordinates. I'm not sure whether these are necessarily off-limits - I can find nothing which makes French legislation public domain (as is the case in e.g. the USA) and yet we have these: osm.org/browse/node/479324518

Would anyone think that a list of lat/lon pairs listed in an Order-in-Council is subject to copyright, or would they amount to "bare fact"?

(Of course, the above is nothing next to the heinous crime of drawing the Irish boundaries *through* Lough Foyle and Carlingford Lough.)