I cannot speak authoritatively on what the make up of a garrison town was. I would imagine that those that were along the coast and had forts or castles have magazines and arsenals within the walls. Barracks were being built in many of the towns and I would again suppose that they contained arsenals, magazines, etc. For a look at the subject see, "Ten British Military Towns", edited by Peter Dietz, Brassey's Defence Publishers London 1986. I am not certain how much of it deals with our period, though.
The figures I quoted are only for those regiments which were serving with Wellington's army and not for the other regiments at home or those which had battalions overseas in other locations. The figures represent the numbers deemed trained and available to reinforce.
The district commanders had all of the regiments, depots, yeomanry and embodied militia assigned to their districts under command. Many of the units were organized into brigades, but I do not have that information. The militia appear to have had 28 to 31 consecutive days training a year in camp [plus whatever local training was required?] when not embodied for service. However once embodied for active service, I would imagine that they had full training periods and there were a number of ad hoc camps formed for training. Each district had Inspecting Field Officers who regularly inspected both the regular recruits, yeomanry and the militia for state of training, etc. During the War of 1812, in the U. S., the city of Baltimore militia trained for two weeks per month at least from 1813 and the New England militia were considered the best trained and armed in the U. S.
While service in the militia spared you impressment, Parliament allowed volunteering(!) from the militia into the regulars during certain periods of the year. It is noted that on 30 September 1813, Wellington's army had 54,935 men, almost the exact number of militia men who had volunteered into the regulars since 1808. Between January and September 1809 approximately 48% of the recruits from the British Isles came from the militia.
1. Severn District: Gloucester and the vicinity of Bristol, Worcester, Hereford, Monmouth and South Wales. In 1810 Stations were Bath and Bristol. In 1811 and 1812, just Bristol. Lieutenant General Tarleton in command until about July 1810.
2. Dundas commanded the Northern District with H.Q. at Newcastle on Tyne. By July 1812 he is replaced by Lieutenant General Sir Charles Green. There were one or two subordinate Major Generals on staff there as well.
3. South West District under Major General Arthur Whetham. Stations were Portsmouth and Gosport with Weymouth added in 1810. District was Wiltshire, Dorset, Hampshire and Portsmouth and its dependencies.
4. Home District was commanded by HRH Duke of Cambridge and Sir Harry Burrard [then Burrard alone by 1812]. H. Q. was in London. It consisted of Middlesex, Surrey, Hertfordshire, Berkshire, Kent to the River Cray and Holywood Hill inclusive with part of Essex. Eastern District was H. Q. at Colchester, Kent District H. Q. at Canterbury.
5. Not certain what he refers to. There was the regular county militias and yeomanry, embodied and assigned to the districts, but many served outside of their own county. Then in 1808 the old volunteer movement was reorganized into the Local Militia and perhaps this is what he refers to. They could not serve outside of their county if I recall correctly until 1813. There were 179 regiments in England, 24 regiments in Wales and 67 regiments in Scotland. The quota was set at 213,609 men.
Hope this helps.