It depends on what you are looking for - if you are looking for one for your laptop, its unnecessary because laptop will convert 220 to 110 (just check your power source to make sure that there is a built in converter)
As for plugs, there are standard British outlets here - so if you plan on bringing anything, bring that.
If you are planning on bring other appliances, be wary of being taxed when you enter the island - they can and will tax you on all appliances that you bring. That includes alarm clocks and coffee makers, etc.
Other than that, just bring the British converter plugs so that you can plug standard US plugs into wall sockets.
My suggestion would be to go to Radio Shack and get the necessary equipment - they're usually pretty helpful - just tell them what you are looking for.