I envision it as more like a “withholding” where you have no choice, the maximum possible taxes will be taken out, then you have to submit documentation to get your own money back.
Discussion
fortunately there's lots of options for offshore bitcoin<->fiat exchanging. xapo bank is based in gibraltar and they know they'd probably lose most of their customers, and the UK govt would have a hard time trying to force them to become tax collectors for them. but i suppose it may happen at some point. i won't be a customer at that point, if it becomes an EU thing and they kow tow to it.