Interesting question. You are allowed to be outside of the place where you are living in order to return home, but I take it that St Lucia is not your home, so as I read it, the exception for returning home would not technically allow you to go to St Lucia.
Are you going there for work, or to fulfil a legal obligation, or to visit a friend or family member in hospital / death bed / jail, or for a wedding / funeral / childcare, or to view / manage residential property, or because you are going to make St Lucia your home? Those are valid reasons to be outside the place where you are living.
My understanding was that this was only applicable to UK residents - who British or otherwise - are bound by those guidelines.