Royal Lancashire Militia. Numbers and names of regiments changed over time. The following existed at the time of the wars against Revolutionary France and Napoleon:
Royal Lancashire (1st Regiment) Militia
1st and 2nd Supplementary Militia Regiments. These later became 2nd and 3rd Royal Lancashire Militia Regiments. The Third was retitled The 3rd Royal Lancashire Militia (The Prince Regent's Own) in 1813.
3rd and 4th Supplementary Militia Regiments became 4th and 5th Royal Lancashire Militia Regiments. These last 2 disbanded in 1799 and their men were absorbed in other units.
1st Royal Lancs recruited in Fylde and Lancaster areas. 3rd Royal Lancs in Garstang and Preston areas and Leyland Hundred. (I've only listed recruiting areas within easy reach of Cockerham, where we know John Whiteside was post-war.) Throughout the Napoleonic Wars the Militia provided drafts of officers and men for the Regular Army.
A Supplementary Militia ("The New Militia") was raised during the Napoleonic Wars and disbanded 1816. I'm not sure if this was the same as 1st, 2nd, 3d and 4th Supplementary Regts. mentioned above.
There were also Local Militias and Volunteer Forces. These remained in the county and were for local protection.
3rd Royal Regt was embodied 1798-1802 and again 1803-1816. It served mainly in England. It was in Dublin from 1813 and remained there after peace with France was declared May 1814.After Napoleon's escape from Elba and return to France, so many officers and men volunteered for the Regular Army that the Regt was reduced to less than half-strength. Many of the Lancashire volunteers fought at Waterloo.
I believe, from earlier research, that the 1st Regiment was also stationed in Ireland around 1815.
See:
Handlist 72 Sources for the history of the Militia and volunteer regiments in Lancashire (Lancashire Record Office) This is not an exhaustive list.
Lancashire Infantry Museum- The Duke of Lancaster's Regiment
www.lancashireinfantrymuseum.org.uk/the-royal-lancashire-militiaSoldiers, Sailors and Strangers- Baptisms 1800 onwards (web page) From baptisms on this it seems that 1st and 2nd Regiments , Lancashire Militia were in Northumberland around 1799/1800. Some soldiers brought wives from their home county, some married native girls, some had met their wives elsewhere in Britain. All good for the gene-pool! E.g. Son of William Gardiner, a native of "Cockron", which I guess was Cockerham, serving in Lancashire Militia, and wife from Cumberland, was baptised, aged 2 or 3 at South Shields.