That's not good job from Amex.

Plat users should have the right to use the local Concierge, who knows local issues better than a remote one.

Instead of re-routing the call, they should re-route the costs (internally). It's clear that the UK Concierge earns nothing from your US fee, but it's not a reason not to serve a customer. Amex is a worldwide company offering worldwide services. The customer should talk to every affiliated company like he talks to it's local representative.