Why does the name never globalise into the British name?
They do. The Twix was called Raider across Europe until 1992.
The story behind Snickers/Marathon is this. The product name was actually registered in the UK first, with plans for a worldwide launch within the next few months. Unfortunately while the launch was being planned another brand registered the trademark Marathon in the rest of the world; hence the product became Snickers in the US and Europe.