If it really does work out cheaper, buy two lots of returns, each with the return leg next year. For example:
London-Barbados 10 November 2010
Barbados-London 18 October 2011
Barbados-London 16 November 2010
London-Barbados 12 October 2011
Carriers vary as to how far in advance you can book but its often 355 days (from day of booking). If you end up not being able to use the return legs of both tickets, you've still saved; if you are, you're quids in.