> That’s also very common in the UK

Touristy places in central London maybe. They wouldn't dare try sneaking an undeclared service charge onto a locals bill in Northern England or Scotland :-)

> In the UK, they are legally but not practically or socially discretionary.

Speak for yourself. When entertaining clients at touristy places in London, one routinely asks for this underhand charge to be removed from the bill. (Whether or not we then choose to leave a cash tip is a matter of discretion, but the chances are rather low after such shenanigans).