The marriage in Kenya was legal in Kenya, in Britain, and in the US. Therefore, he could not get legally married in the US.
Had he taken Ann to Kenya to marry her there still would have been legal issues concerning that marriage as she was white, he was black, and he already had a Tribal marriage.
There was documentation to the marriage? Because I really question the legal in Britain thing. If no marraige was filed legally, how was it legal? From every account I have read, it was strictly a religious ceremony with no legal filing. It was the islamic way to get around polygamy laws they lived under when they were still under British law/rule.