You did a search for flights in WT+. The computer is telling you that unfortunately, WT+ does not exist on all the flights on the itinerary proposed. It's a bit as though you were looking for flights in F between London and Manchester, there is no F cabin on such flights so any booking engine will suggest to you that you book the closest alternative which is C. If you choose to book it, you won't get the F TPs just because you were looking for F in your search, nor will BA create a special, different and higher price for people who inadvertently looked for F and will book C.

There isn't and has never been any premium economy on short haul BA flights. It does not exist. There is therefore no fare that could possibly be created for BUD-LHR-TLV return with all segments in WT+ and consequently nothing to discount when the flights that were always going to be in economy are indeed in economy. Incidentally, you are booked in K on those flights precisely because that fares specifies that when the long haul is in T, then short haul segments will be booked in K (E would have short haul in B, and W would have short haul in Y).

You are getting what you are paying for and to use your word, logically, you are getting the TPs for what you are flying.