Currently Yahoo Search in the UK is still running off the original Yahoo index and algorithm.
As you know, Bing is powering Yahoo's organic and paid results in the US and Canada. Last week, Yahoo and Bing announced they migrated over the organic results in Australia, Brazil and Mexico to Bing.
So what is next?
A WebmasterWorld has some speculation that Yahoo UK is next. Some webmasters and SEOs are noticing that the Yahoo UK results have "two different indexes" on refreshes. By that, if a person searches once and then again, they are seeing two completely different set of results. In addition, when they compare one of those sets, it seems to match up with the Bing results. This shows me that Bing and Yahoo are heavily testing the UK migration and it might be next up on the list.
Are you ready for Bing to power Yahoo in the UK? I assume so, at least by now.
Forum discussion at WebmasterWorld.