I'm not suggesting it's acceptable by UK standards, but is 15-20 minutes so unusual for a US call center? Most US customer service lines I've called have kept me waiting for that kind of time - apart from United Airlines who kept me on hold for an hour and 45 minutes last time I need to speak with them urgently.
The SB hold time target is FAR lower than that, but I guess they're undergoing some difficult restructuring at the moment.
The wait time for the UK line pretty much depends on whether David is at his desk, or making an espresso when you call.
Rob