At the 1444 start, there are only three nations that meet that criteria:
England Scotland The Isles (Scottish vassal)
Of them, England and Scotland are recommended. They have a special method of forming Great Britain.
Requirements
There are two decisions for forming Great Britain. They both have these requisites in common:
At least Admin Tech level 10 Not a subject nation Not at war
Note: You usually get Admin Tech 10 at around 1531.
Method 1: Form British Nation
This basic decision has these conditions:
London is part of a state Own these core provinces: London Oxford Hampshire Lothian Aberdeen
Method 2: Form British Nation Diplomatically
This variant is exclusive to England and Scotland. These are its conditions:
Has 100 Legitimacy If you’re playing as England: Scotland must be a vassal or a junior partner Scotland must have less than 15 provinces If you’re playing as Scotland: England must be a vassal or a junior partner England must have less than 41 provinces
The subject’s loyalty doesn’t matter. You’ll automatically annex/inherit them when completing this decision.
Strategies as England
1. Complete the “Levy the Troops” mission
This mission needs you to:
Reach 100% land force limit Have at least 60% manpower reserves Have at least 1 general
You already have a general at the start. You can complete the other two conditions within the first week of the game by doing these: You’ll get these rewards:
“Subjugation” CB over Scotland Permanent claims on the Scotland and the Ireland regions
The CB expires in 25 years though. You may want to hold off on claiming these rewards until you’re ready to declare war on Scotland.
2. Subjugate Scotland
It’s recommended to defeat France first before you attempt to vassalize Scotland. They’re guaranteeing Scotland’s independence at the start. When you sue France for peace, demand that they annul treaties with Scotland to remove their guarantee. Once Scotland’s on their own, you can then declare a subjugation war on them. You can easily siege down their capital and occupy their provinces. They’ll bend the knee eventually.
Strategies as Scotland
Unfortunately, your missions don’t give you a “Subjugation” CB over England. You’re rewarded with permanent claims on them instead. Secondly, England’s size makes it costly to vassalize them as an unjustified demand. You’ll need to annex some of their provinces first before that becomes possible. That said, vassalizing them to form Great Britain diplomatically is still more efficient than the basic method. You’ll save up on admin mana and you won’t get any aggressive expansion. For a more in-depth guide on playing as Scotland, refer to this guide.