It kinda goes both ways- sure, the UK can make a power play here, but Uber can make an example to drivers in other countries as well.

"You want to make a case that you're employees? Joke's on you- now you have nothing. Peace."

The Wal-Mart analogy is actually pretty good- they've effectively deterred workers from unionizing by simply shutting stores where workers try to form a union. They've made it clear that if workers form a union, they'll shortly be out of jobs.

One can argue about the morality of this approach, but not really its effectiveness.