Q: How to let the British Island join the HRE?

[Trin Tragula]

Considering the AI practice of disbanding it's entire navy when their ship goes obsolete I'd say the strait might unbalance things if you're unlucky

 

[Trin Tragula]

It's admittedly before the game's time frame, but I think it's worth pointing out that Richard, Earl of Cornwall (second son of king John - lived 1209-1272) was elected King of the Romans in 1256.

There's a rather big difference between that and joining England to the empire. The game models joining the wrong way around anyway. A province joining the empire represents your own monarch actually giving up some rights of sovereignty over it. It should be a very bad deal, very much contrary to the entire project of centralizing power to the state that is the goal of most states during this era. Certainly nobody tried adding provinces to the empire during the entirety of the Eu3 timeframe...