Since BWI is a dependent area of the UK, and the islands in the BWI are not listed separately in the treaty list ( at least in Pub 515) it appears you would need to apply the UK rules, which are:
You must withhold 10 percent federal tax, unless the beneficiary chooses to waive the federal tax withholding, at which point you apply the UK treaty rate ,under the NRA withholding rules, which is zero. If the beneficiary waives the federal withholding, he/she must provide you with a completed and signed Form
W-8BEN Certificate of Foreign Status of Beneficial Owner for United States Tax Withholding.
See IRS Pub 515
http://www.irs.gov/pub/irs-pdf/p515.pdfIf the beneficiary waived federal withholding and is therefore subject to NRA withholding, the distribution is reported on Form 1042-s , not Form 1099-R
If the beneficiary is unable to obtain a SS#, he/she should apply for an ITIN- you need this for your reporting purposes. See
http://www.irs.gov/individuals/article/0,,id=96287,00.html