I'm inclined to go with Martin who originated the thread and Malcolm's interpretation, given that Martin came back and agreed with him having said originally that it was for a project.

On my pc Malcom says "the official march of the county regiment is as close as you'll get" which is at odds with "has decreed that is has to be some boring regimental militarism"

So - suspending the wit and imagination for this post only - I would suppose that a significant proportion of a county's population would at least recognise the song - not just us folkies, and when they heard the song they would feel that it belonged more to them than to people in other counties

It follows therefore (anyone still awake?) that people from outside the county are not in the best position to say what it should be. So not being best qualified for any of these counties these appear to me (note - Appear To Me) to be the best contenders so far

Cornwall - Trelawney
Tyneside - Blaydon Races
Dorset - Dorset is Beautiful
Somerset - I am a Cider Drinker (based on the number of requests when playing there)
Yorkshire - Ilkley Moor
Derbyshire - The Derby Ram
Lancashire - She's a Lassie from Lancashire (?)
Cumbria - John Peel
Sussex - Sussex by the Sea (based on it being adopted by Brighton and Hove Albion
Lincolnshire - Lincolnshire Poacher
Devon - Widdecombe Fair (by popular selection as per link in the first post)

and entering the realms of suggestion -

Buckinghamshire - The Eton Boating Song