I'm going to give you an "informative" for that - the last 2 paragraphs especially.Yes indeed, the Militia and Yeomanry (and for a time, the Volunteers, Fencible Cavalry, Fencible Infantry, Sea Fencibles and Provisional Cavalry Regiments) certainly did have a place in the defence of the home country - especially in Ireland, where they were very much a part of the orbat for operations against rebels and the French - and again in Wales in 1797, where Yeomanry, Militia, Volunteers (including some hastily-raised Volunteer units) and regulars (i.e. RN and Customs Service) served as part of the same formations.
The Lord Lieutenant was theoretically in command of the county's forces, but Embodied Militia regiments were regularly posted to the far end of the country and placed under regular Army control. The Yeomanry were also posted away in this manner in the latter part of the Napoleonic Wars, so there must have been a centralised mechanism for requesting Militia/Yeomanry support from county Lords Lieutenant and seconding them to supplement and replace regular Army units on garrison duty. For example, the garrison of Bristol in 1797 under the command of the regular Army General James Rooke, comprised the 13th Foot, the Royal Berkshire Militia and the Suffolk Provisional Cavalry (which were a short-lived sort of conscripted Militia Cavalry). When this force was mobilised to march on Fishguard, the local Volunteers and Yeomanry were mobilised to become the new garrison in their absence. The garrison of Windsor Castle for much of the Napoleonic Wars was provided by the Royal Staffordshire Militia (much praised by the King and Prince Regent for their smartness of turnout, bearing, band and ARTILLERY detachment!), thus freeing up Foot Guards for ops.
The Volunteer Corps were privately raised and funded, but it was on much the same terms as the Yeomanry. Their formation had to be authorised by the Lord Lieutenant and his county committee for Militia and Volunteers. The state would then provide small-arms and equipment, with all other expenses being met by the corps. Officers were commissioned by the Lord Lieutenant and entered on to the Militia, Yeomanry, Fencible and Volunteer edition of the Army List. As mentioned, six months' good service resulted in the removal of a man's name from the hated Militia Ballot, which does go some way to explaining their huge popularity during the Napoleonic Wars. Some of the better ones were raised as an adjunct to the county Militia regiment - often providing a light infantry component. Many of these had been raised earlier during the American War of Independence and were re-raised in the 1790s.
Chains of command were 'complicated' to put it mildly... At Fishguard, a long-serving Lt Col of Militia and the local Regulating Captain of the RN were 'gazumped' by Lord Cawdor (a Yeomanry Major) due to social 'rank'. There was also then a colossal argument (settled by a later duel) when Cawdor's force met the retreating Fishguard & Newport Volunteer Infantry, whose CO held a Lieutenant-Colonelcy senior to that of Cawdor... I can easily imagine the chains of command at Peterloo to have been equally as Byzantine.
But I'm only doing it because I can't find the "Fvcking brilliant" button
STONKERNOTE: Just out of interest, around 1999/2000 in the wake of a fatality during an ACF night exercise, together with a colleague, I was tasked by HQ LAND with identifying the fractures in the Regular Army/ACF CoC, which led in turn to looking at the history of linkages between Regulars and TA since the ACF were formed. This added another layer to my undestanding of the TA/Regular relationship, which I had previously investigated up to a point as a Training Major a few years earlier. You've just now added another layer to my (still far from perfect) understanding of this area, and deepened my conviction that - however 21st Century we think we are - every step forward taken by our our Army is made with long tendrils of history wrapped like brambles around its metaphorical ankles, impeding all efforts at onward progress