It is normally the case that it is less expensive to buy WT+ and upgrade to http://www.newclubworld.com than it would be to purchase WT and upgrade to WT+.
This is because you cannot upgrade the very least expensive WT fare buckets; whereas you can upgrade the very cheapest WT+ sale fares to Club.
Semi-flexible Economy is often more expensive than inflexible Premium Economy.
Given the difference between a Premium Economy seat and a Club World fully flat bed (including departure and arrivals lounge access) is significant, this is another reason why when redeeming or “MFUing” (=upgrading with BA Miles) you should only ever do so into Club and First.
Think of how many poor souls spend all their hard gotten miles upgrading to Premium Economy….