The best bet, simply due to (relative) proximity, would be the Line Islands, today part of Kiribati. They are just south of the Kingman Reef and Palmyra Atoll (which are of course American) but they really are the closest bit of territory available. If you have a timeline in which Canada gets a few minor colonies in the Atlantic (aka Bermuda, the Turks and Caicos Islands, etc) from London, I could see the Brits also ceding those islands in the Pacific to their friendly dominion.
However I'd say this is MUCH easier with a pre-Oregon Treaty POD, since it'd enlarge the Pacific coastline southwards. But since this is post-1900...