"Book and upgrade" (sorry, not "buy and upgrade") is different to buying and upgrading later. "Book and upgrade" is a one step process through the BAEC pages.

I've just been to the website, and you can "book and upgrade" (both ways) a WT to WT+ ticket LHR/JFK for £711 + 25000 miles. Sept 9/23rd 18:05/20:40

It's £416 for the cheapest economy fare (interestingly, that is changeable for £90 up to 1 day before travel - N/S bucket). Upgrading that to increase flexibility makes it £1215.

A WT+ ticket would cost just under £900.

Obviously, there are substantially increased taxes (and fuel duty) on a WT+ upgrade, so I suspect that the "book and upgrade" is from the lowest fare. Still poor value though - I would spend the extra and get the TPs - plus the possibility to upgrade to CW if availability increases nearer the time.