As I posted earlier, it is plain from FCC's engineering works posters that
they do indeed have the capacity to turn trains at St Pancras
International (low level platforms).

I did have a chuckle at one piece of propaganda that said there have been 16
sets of points upgraded between Kentish Town and Kings Cross. Could anyone
name them all?

AFAIAA there will be no turnback facilities at St P. International LL (no
signals and no pointwork). Given that any future work or blocks will be
south of Kings Cross Thameslink station, I can only imagine that all
turnback trains will just travel empty to KX and then use the pointwork that
was originally used for trains terminating there from the south during the
original blockade of Kings Cross. I can see a split service of trains
terminating at Kentish Town and some terminating at St. Pancras