The original question was concerning the last white American/British boxer to hold the heavy-weight title.
As stated earlier, Joe Bugner in the late 90s held a "joke" version of the title. Here's the relevant bit from Joe's Wikipedia entry.

He[ Bugner}made a final comeback during the 1990s, winning the Australian heavyweight title in 1995 and the lightly regarded WBF heavyweight championship in 1998 at the age of 48 against Bonecrusher Smith.

Of course, Joe may no longer be a British citizen.