USB power is USB power. That part of this discussion does not vary by country.
The physical charger plug necessary to connect to the socket can (does) vary.
For the Apple chargers using the “duckbill” connector, Apple offers a plug kit: Apple World Travel Adapter Kit.
About the Apple World Travel Adapter Kit - Apple Support
Saint Lucia uses British plugs, and 50 Hz 240 volts. Apple chargers all support that, which means you need “just” the plug.
You don’t need a power converter.
Or you can purchase a British USB-A or USB-C charger when you get there whichever you are using now) and bring your cable with you, and which might be cheaper. Cheaper, if you don’t buy at the airport or in tourist-focused stores, that is. (Or you can tell the front desk at the hotel that you don’t have one, and ask them if they have a charger in their lost-and-found bin. This will not be the first time they’ve heard about plugs. Some hotels might have adapters in the room.
Want to bring your own? On a major online market vendor’s site, I see a US to UK Plug Adapter, Ireland Scotland Type G Travel Adapter, [brand] Power Adapter with 3 American Outlets and 2 USB offered for ~USD$17, which’ll allow you to use a USB-A cable with your iPhone. This entirely replaces your charger, if you have a USB-A to Lightning cable. You don’t need to plug your charger into an adapter that also offers a USB connection. There are probably also USB-C chargers offered, too.
Again, USB power is USB. Just know which type of USB connector you have on your charger cable; USB-A (rectangular) to Lightning, or USB-C (rounded) to Lightning.