I had a friend in the U.K. with a similar trip...This is what he did

A) Flew STN JFK-STN with AA where fares are cheap

B) Started a Circle Pacific in JFK and went JFK South America-Australia-Asia-JFK and it was less for two reasons

THE Circle Pacific is less than the RTW
The Circle Pacific starting in the U.S. was priced in USD which is way lower than the Euro and certainly lower than the GBP fare