Whilst exploring the internet I came across this book:

http://www.amazon.co.uk/Salt-Routes-Old-Photographs/dp/1848684762

and reading the authors blog, I discovered that salt routes were some of the earliest roads criss-crossing the UK, as salt was traded between towns.

From wikipedia: A salt road (also known as a salt route, salt way, saltway, or salt trading route) French: Route du Sel) is any of the prehistoric and historical trade routes by which essential salt has been transported to regions that lacked it (see History of salt).From the Bronze Age (in the 2nd millennium BC) fixed transhumance routes appeared, like the Ligurian drailles that linked the maritime Liguria with the alpages, long before any purposely-constructed roadways formed the overland routes by which salt-rich provinces supplied salt-starved ones.

This makes a clear connection between salt and migration, as salt was the intial reason for the laying down of many routes of trade and therefore of migration. I have ordered the book and am excited to find out where the routes went..