i guess if someone lived in wales and understood the passage of trade they'd be aware of the cambrian hills. a long stretch of hills. this low ground one side, that low ground the other. maybe there'd be a few people in that region who could put their local knowledge together into a sense of the whole range.

maybe once every now and again they'd meet up with the people who knew about the mersey, and the penines.
"where does your region go?"
"er, kind of that direction. we've got this long ribble valley, and then turn left at leeds for a couple of weeks' walk. there's this wierd group of guys over there who know where to go then."
"do they need any tin? what do they grow? do they have someone who can cure a bad back or a storyteller?" etc.

then they'd all know a little bit more about the island, and each other.

it isn't a map of the coast. what would that be for? it is a route map, showing major routes and an administrative centre.
how strange to think the people who aligned countless rocks over europe with the movement of stars, and built stonehenge with stones of far-flung origin, didn't know where they were going. over the roman, saxon, norman etc. invasions, there seems to have been at least a tiny bit of loss of technology.

perhaps ireland isn't included because it's not a map of ireland.