You could book this tomorrow with travel good for a year. Here's one (basic) route -
click here for details.
In economy starting in the UK, this would cost GBP 2100 plus fees and taxes, 25% less for kids under 12. In business class the price jumps up to GBP 6200, but since RTW tickets are priced differently depending on where you start and end, you could save quite a lot on the business class option by starting in someplace like, say, Norway (GBP 5295.) You would need to do some research on the "best" options for you once you're ready to pull the trigger.
The tickets are good for a year and offer other advantages like a fairly liberal change policy, baggage is included, etc.
As mentioned, the above map shows 11 segments out of a total of 16 available, so you could travel more extensively in North America (which includes Central America as well as the Caribbean) or perhaps include New Zealand during your time in the antipodes.