As a PLT nearly 3 years ago, I was op-up from WT+ to CW on LHR-IAD, when I inquired in the Terrace lounge in LHR. Of course that was on July 4th or something like that, so I may have been lucky.

As for WT+ being a "buffer", I'd rather think it's a way for BA to attract people who are willing to pay a bit more for some extra comfort, but can't afford the CW fare. But I wouldn't be surprised if this "buffer" thing was one of many factors behind the WT+ concept--but they'll never admit it.