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The British army - which is made up of a number of regiments
Infantry Regiments (30)
85 battalion
In contrast to the Royal Navy, Royal Marines and Royal Air Force, the British Army does not include Royal in its title. This is because, historically, British Armies comprised individually raised regiments and corps. The position of the British Army as the Army of Parliament and not the Crown was confirmed by the Bill of Rights of 1689 requiring Parliamentary Authority to maintain a standing army in peacetime. Nevertheless, many of its constituent Regiments and Corps have been granted the "Royal" prefix and have members of the Royal Family occupying senior positions within some regiments
As far as I know, all regiments in the British Armed Forces are similar in size.
Three army corps of the British Army fought in the Dardanelles Army. It was formed in 1915, but only lasted a few years.
Some regiments did and some did not. It apparently depended on their role at the time.
In the US Army there were four regiments to a division in WWI. There were two brigades in a division, and each brigade had two regiments. US divisions were huge compared to those of every other major combatant, with about 26,000 men. The British system was more complicated. They have a number of regiments with a long and distinguished lineage and history. But British regiments did not fight as formations. Each British regiment has a number of battalions. In the British system three battalions, almost always from different regiments, were brigaded together. Three such brigades made up a British infantry division.
The Royal Artillery is responsible for the firepower of the British Army and copious regiments across the United Kingdom and a few in the North-Western region of Germany. According to the British Army website, the Royal Artillery supplies the British Army with radars.
Black regiments
The British Army are no longer on the streets of Northern Ireland. Most regiments have left, with only a very small amount still remaining in barracks.