> with a CBDC is that there is nowhere left to "withdraw" to

Why can't I use them to purchase dollars or yen? How did we get from the BofE issuing a currency that people can use to everyone being forced to use it? (Also, may I humbly suggest the wikipedia article on Gresham's Law, if you're not familiar with it: https://en.wikipedia.org/wiki/Gresham%27s_law)

I agree that bad things would happen if everyone was forced to use a currency they don't want to use, but that's kind of axiomatic.