Longbowmen  => Rangers  => Green Jackets  (skirmishers, upgrades to line infantry. The Longbowman has an enhanced rate of fire but is costly, while Highlanders are so tough they can make for line infantry. The ultimate upgrade, the Green Jackets are on par with Native Americans, with the ability to move stealthed and with no min range, but are costly. These units lose their abilities upon  however)
Redcoats  => Regimental Redcoats  => Colonial Rifles  => Old Contemptibles  (line infantry, dressed in the traditional red garb, with good damage and armour, and like the longbow, with a withering rate of fire that causes lesser mortals to melt...but a compensatory pricetag)
Foot Guards  => Grenadier Guards  => Royal Marines  (a vastly superior grenadier with a higher pricetag. Grenadiers are as good as the Royal Musketeers, although by  the line infantry line will lose minimum range)
Black Watch [3 & 4] => Gordon's Highlanders  Excellent, all-rounder infantry - can't entrench like Marine units, but grants a balance between the power of grenadiers/marines, the range of snipers/marksmen and the cost of line infantry.
Yacht  cheap and strong light vessel of the Colonial Era.
Rocket ship  shared with India, fires rockets that set ships on fire. Replaces anti-ship units.
Cathedral - performs taxation and religion research, and is the main stopping point for your auxiliary units as well as chaplains, which have mediocre attack, but super defensive and defence-boosting skills
Reduces the build time of ships, foundry units and aircraft
Has a +10 oil and +10 metal bonus (once 3rd Green researched).
Reduces the knowledge costs of science research and the research time for military research.
Grants access to upgrades that allow for the creation of dreadnoughts, howitzers and heavy machine guns in .
Has a facility for building recoilless gun infantry and tanks (different non-European sides build different tanks in  - Tanks are powerful mechanised units that destroy all other units, but have their own weaknesses - recoilless guns destroy them; they are vulnerable from aircraft attack; a direct hit from a cannon could potentially destroy them; and they are slower than tankettes, although they are much powerful in a shot-for-shot battle.)