The challenge here is that the actual LGW 777 airframe is vulnerable to change, often at the last minute [1-2 days out]. I don't think we've
ever done a Caribbean trip without having at least one sector being changed, which makes seat planning almost a waste of time. indeed, on 2 successive trips to KIN, we had all 3 versions!
We're just home from an Antigua trip [in J], which ISTR is the one that connects onward to St Kitts. Outbound we ended up with a 4-class, with the F cabin being used as J for 10 lucky people. Inbound we had a 3-class, but that was changed a couple of days out from the 48J to the 40J version, leading to a frantic change of our seats as BA had put us in 11D/E [aisle and middle]. We moved to 4J/K [window pair with aisle access].
So whatever you choose, keep checking your plans haven't been silently undermined by BA ... because they won't tell you it's all changed!