Commonwealth

Allied Kingdom (連合王国?), capital city is Londonium, is a small nation with more than forty colonies in the alternate world. It stayed neutral at the beginning for doubting about whether the Republic would capture the Western Industry Area of the Empire, and hence defeat it. However, it was searching the chance to join the war after the Republic was pushed back, and finally did it when the Empire rejected its requirement for “restitutio in integrum”.

International Relations
Unified States (合州国) - Due to lack of personnel, Allied Kingdom signed up an agreement to US to share knowledge, technology and volunteers to tear apart the Empire army.

Francois Republic (フランソワ共和国) - They signed a pact where Allied Kingdom will provide intel about the military and economic data of the Empire. Lending resources and provide ship cover in English Channel.

Rus' Union / Russy Federation (anime) (ルーシー連邦) - Because of the different ideology, they treated each other hostile before the Union declared war on the Empire. However, after the first invading army of the Union was defeated, they started having diplomatic exchanges, such as exchanging the arrested spies, enabling Arctic convoys, sending expeditionary force, weapon and material supply.

Commonwealth Nations - The nation needs a lot of manpower. They thought a good idea of using their former colonies to give aid and replenish their ranks.

Army
The Allied Kingdom has a third level of advanced military with limited trained soldiers. However, they can only use guerrilla tactics and used some camouflage to hid themselves. They are still no more much to the Empire's forces. When Operation Revolving Door was performing, they only had 7 infantry divisions and 1 cavalry division on main islands to send abroad, which was just a drop in the bucket compared to the Empire.

Navy
They have large amount of heavy and long-ranged battleships, containing some super-dreadnoughts. They have the more advanced and powerful mechanized ships than the Empire. Their tactics of naval mages are the most advanced, such as taking off from submarine, ship-borne attack, and short-range air support against the marine corps.

Airforce
They have maximum air power and resistance and advanced long-ranged fighter planes. All of their pilots are elite and highly-trained personnel. Therefore, they put a lot of effort to maintain the air superiority in the sky. They are one of the strongest in air superiority.

Spy System
Allied Kingdom is one of the few countries who used knowledge, deception and gathering information about the enemies. They even broke high-level encrypted radio and teleprinter communications of the Empire, which had a special code named Ultra.

Trivia

 * On the maps that show up in the background of the anime, the country's name shows up as "Commonwealth" in a highly stylized font.
 * In the manga, the full name of Allied Kingdom is Allied Kingdom of Albion.