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And how do I exit this walled garden and pay in GBP to UK, or USD to USA, or dare I say Yuan to China.

What about RSD to Serbia? CHF to Switzerland?

You put the money in your bank account and use that to pay, or you buy the other currency. The same way you would with physical coins and notes.

Or, if the UK/USA/China set up their own pseudo-cryptocurrency, you can probably exchange digital euros for digital dollars or digital yuans.

you do standard FX... what are you trying to say?