Firstly, as I’m sure you know, airframe changes on Caribbean routes are endemic. No doubt a dummy booking on your part would reveal whether BA is actually selling F on your flight. If that’s the case, I assume you could potentially pay to upgrade to F. If not, then presumably the F cabin might end up being used as Flub … which then generates a discussion that’s been going on here for several years about whether to select a Flub seat or not.
My gut feeling, from other posts here recently, is that demand for F to UVF is such that it now justifies a 4-class. Likewise, our imminent LHR-ANU is a 4-class (Club Suite, 1-2-1). I have no feeling for the state of F cabin upgrades; J suits us just fine!