I need to fly to LGW, I will take BA for my transatlantic flight. I cannot fly to LHR because of onward connections from London. My flight choices from NA to LGW are
St Lucia - LGW
Tinidad and Tobago - LGW
St Kitts - LGW
Barbados - LGW.

I have not been to any Caribbean islands. The beach of no interest and I am wondering which destination I should choose. What should I do:-

go to the destination that will give me the most miles?
travel to the most expensive destination?
travel to the destination that has the most frequent AA and BA service?

Any advise will be appreciated I will only spend two or three days in whichever island I fly to.

Thanks.