The Crimean War showed that the British army couldn't defend an expanding Empire, protect the homeland, and undertake major operations overseas. To compensate, the Volunteer Force was formed.
It was to be a part time army. Members were to be returned as “effective” if they had attended eight days drill and exercise in four months, or 24 days within a year, the rest of the year they carried on normal lives. In fact, the Volunteer Force system slotted neatly into the British class structure of the time. The regular army had working class troops and upper class officers, the Volunteers tended to attract members from the expanding middle class, who could afford the equipment and had enough flexibility to take time off.
Units were Rifles or Artillery, and (at the start) remarkably autonomous. Originally small units of about 100 men, they were integrated into battalions during the early 1860s, and they also started to receive a grant to defray expenses! The military role of the rifle corps was to harass the invading enemy’s flanks, the artillery corps were to man coastal forts and defences.
Uniform and Arms
There was actually a recommended War Office uniform, though it wasn't compulsory (a fact seized upon by various county associations around the country). The tunic was brownish grey, with green facings, and the cap grey with red braid. The trousers were the same colour with red cord down the sides and russet gaiters, into which the trousers could be tucked to form knickerbockers. The was also a greatcoat with hood.
In practice, many prefered to chose their own uniform, which was permitted as long as the Lord Lieutenant* of the county agreed, and so many consequently adopted green uniforms, based on the rifle brigades of the Napoleonic war**. By the mid 1860s Lord Lietenants could enforce a common uniform within a county. Volunteers had to provide their own equipment, though they were supplied with government issue "short" Enfield rifles until 1870 when they started to receive the Snider breech loading conversion.
One consequence of the formation of the Volunteers was a huge increase in the popularity of rifle shooting as a hobby, encouraged in the Volunteers of course as a way of increasing proficiency. A National Rifle Association was formed and regular events were held on Wimbledon Common, before moving to Ministry of Defence land at Bisley in 1890.
How useful they would actually have been we will never know, the military "what if" The Battle of Dorking suggests not very much. In a stand up fight against regular troops they would have struggled, but in harassing an invader, delaying his progress their role may have significant. After all an invading army had to be supplied across the Channel, which would presumably have meant evading the Royal Navy and making a dash for London. Time and material expended on the Volunteers was that much less available when the Navy closed the Channel again.
* Lord Lieutenants are technically the Queens representatives in each English county, but it is mainly a ceremonial post. None the less they have had some responsibilities over the years.
** The London Gazette lists a Daniel Sharpe as a lieutenant in the Kent Rifle Corps in 1860. A distant relation?