Ryan, 1. The UK never went into the euro zone. To me an interesting issue is Scotland quitting the pound and joining the euro a feasible option for them? Similar to this paper, would an UK-Ireland common currency area work better for Ireland than the euro, but I suspect historical issues with each other make that a political non-starter. 2. I have not yet looked at cryptocurrencies. You are right the data is probably quite difficult to get at. Plus the issue is who holds them, not the people in a single currency area. We ignore a similar thing with the US by pretending the US is the US dollar area in all of our studies when other like Liberia and Panama use the US dollar also. This is probably ok for the US as it is large compared to other economies using the US dollar and the same probably holds for cryptocurrencies for now.

Since the UK voted on Brexit, do you think these tests would favor some other common currency area for them?

Have you considered this approach for cryptocurrencies? The data might be a pain to get though.