I did it as Belgium with historical focuses off. I first went down Government Resigns and Government of National Unity and while waiting for the Collapsing Government national spirit I did the other required focuses for Economic Recovery. To go down the democratic path, when completing the Royal Intervention focus, you have to choose Paul-Émile Janson in the popup. Afterwards, after completing the Adriaan Martens Crisis focus you have to choose to keep Paul-Henri Spaak as your leader to go down the Benelux Union path. Luxembourg and the Netherlands usually do accept. When completing The Council of Europe focus, you can invite other countries into your faction. Here I was quite lucky that the UK had already turned Non-Aligned and disbanded the Allies faction. As a result, I could improve relations and invite them to the faction. With historical focuses off, the UK usually changes ideology and maybe with spies or boosting party popularity, you might be able to get to the same point and invite them to the faction.