Hi Carl,
Not sure on costs, although BA and Virgin Atlantic tend to have a sale in March/April, by which time your flights would be up.
We do UK to BVI each year and would HIGHLY recommend going LGW - ANU and then using VI Airlink to EIS. Prices are reasonable and its not that different to having a private charter. Hassle-free and quick.
We always go Virgin...but its personal preference and depends a lot on which end of the plane you plan to be.