While Dunkirkian Transgressions might well have been a motivator in broadening the horizon of populations to look elsewhere, the more worthwhile question has to be why did those who took to the ships become in any way heroic. Amongst the Welsh Celts, the word "Saeson" was no term of endearment, it was rather a synonym for 'barbarity' and 'pirate' as well as ignorance. Amongst the "Saeson" those who might have agreed with the Welsh characterization would have been titled "treasonous" and not to be trusted. As well, amongst the Welsh Celts, those who considered the "Saeson" a breath of fresh air would probably have had their name entered into the short list.
One answer is a part of the world sometimes called "The
Welsh Marches." It runs basically on either side of a line from the Roman Fort
at Chester in the north, to the Roman Fort at Isca in the south. Think of
it as a piedmont. Foothills between lower land and higher land.
Think of it also as lead mines, copper mines, and iron mines, and if you moved
further west of the line maybe gold and silver and coal. And if geography is not
your favorite, think of King Offa marking a boundary line by raising a
population to dig one hundred and fifty odd miles of ditch. And if the
history of shoveling leaves you numb, think of the Rabbit of Usk, or Saint
Timothy, which is how I try to stay calm.