(oops, again)
I'm still trying to figure out if I could buy a J ticket (or better, one of these reduced tickets) on BA and upgrade to F for 25K points.
You can't. A longhaul J to F upgrade is 50K points return.
But WT+ to J is only 25K return, which is what makes it the real bargain, especially when you can score a WT+ ticket for ~$1300.
But yes, you can book an I, D, C, or J ticket (presuming you meet all the fare restrictions, if there are any) and upgrade to F. If you were only looking at one-way, then it would be 25K points.