London - New York is THE premium business route and business class fares are around 5X economy fares. Fly from a leisure or non business airport, or a route which carries low cost migrant labour, eg Manila - Kuwait, and business class can be had for around 3X economy fare.
Leisure routes are good value for using FF miles to fly in a premium cabin as it allows the airlines to burn off the miles in a seat that would otherwise be empty. Business routes are very poor value with few seats available and max miles being required as the airlines would rather sell these seats for money.
Supply and demand.