According to Steve Brown's article (www.napoleon-series.org/military/organization/Britain/fencibles/HomeGuard.pdf) on the Napoleon Series website the British home forces in 1805, in 20 military districts, comprised the following:
Regular infantry: 50,138
Regular cavalry: 15,684
Regular artillery: 9,310
Militia infantry: 55,818
Volunteers: 212,500
Total (approx.): 343,000
The numerical paucity of regulars is remarkable. For example, in the Northern District commanded by Lt-Gen Sir Hew Dalrymple (of later Cintra infamy), there were just 34 regular infantrymen, all of whom belonged to the 2/25th Regiment; two RA foot batteries; and three troops (183 men) of heavy dragoons. The other 14,500 men in the District were mostly volunteers (11,043), with just a handful of militia.
If one were gaming this, the default assumption might be to assemble a defending army in the same overall proportions as those above, but this would probably be incorrect. There would almost certainly never have been a battle at which 62% of the British force was made up of volunteers, as that would have required the concentration of the entire force in one place. But volunteers didn't serve outside their district, any more than militia served outside the country. So what seems likelier is that the regular element of the overall force would have concentrated, leaving the volunteer forces mustered in their own locales.
The information about the regular units is interesting because, being based on home territory, many are at a remarkably healthy numerical strength. The 1st Regiment of Dragoons, for example, comprised 805 men in ten troops (two troops made a squadron) – something like double the usual campaign strength of a British cavalry regiment. Several line battalions are north of 1,000 men. The 1/95 Rifles were at 809 men, the 2/95 at 763.
Steve Brown doesn't give a breakdown of the volunteers' unit strengths, but I have found one. Looking at the Returns, Presented To The House Of Commons, Of The Volunteer Corps Of Infantry, Cavalry And Artillery of 1806, which someone here pointed me to on Google Books, these report on the numbers, equipment and state of training (of the horses too, in the case of the yeomanry). Under the latter heading, the highest accolade is "Fit to act with troops of the line"; then there is the more qualified "Fit to act with troops of the line with…days' duty", the specified days being anything from three to ten.
Unit numbers varied from fewer than 30 to as many as six hundred or more. The numerical variation in unit size arises usually in consequence of where they are found. Small villages mustered their volunteers as well, but they would be at weak company strength. Towns raised battalion-size formations. Volunteer unit strengths of 500 or more are not unusual. For example, the Wathwood Volunteers of Nottinghamshire, under Lt-Col Walker, are described as having 584 men present (excluding officers), with their clothing described as "Good", and their efficiency as "Fit to act with troops of the line". Some volunteers are called light infantry. The Ashford Light Infantry, commanded by Major Mascall, were 171 strong in three companies, their arms and accoutrements were judged "Good", and they were described as "Fit to act with troops of the line".
Not all of these volunteers can thus be assumed to be likely to join with the army's main body. Many clearly were not up to it. Others might be detached to guard communications, or unable to muster due to enemy occupation of their muster point, even where they were judged fit to act with troops of the line, and so on. But some were up to it, if their inspecting officers are to be believed, so proportionately it seems about 10% of the field army's strength might be volunteer infantry units.
So the picture I am building up is of a defensive army that would have comprised regulars and militia (a bit like Wellington's Waterloo army in fact), plus some volunteers in units "fit to act with troops of the line" and in units, among the infantry at least, of a size similar to regular or militia battalions.
What is less obvious is how to treat these volunteers tactically. One could rate them like Spanish guerilleros, except that these were poor at set-piece battles but good in ambushes. Likewise I don't think Andreas Hofer's men fought in many set-piece battles either. The type of volunteers I am looking at would be in the line alongside militia and regulars. Has anyone gamed this and come up with a suitable treatment?