Respectfully, you submitted a theory. Whilst your theory is based on your copious and broad reaching experience, it's still only a theory until it has some basis in fact. Unless I've totally lost the plot, your last suggestion was a commercial issue is the likely cause of delays and that it may take some time to find a berth in Southampton - one would like to imagine that the Ports website wouldn't show an expected arrival unless a berth could be allocated.
Nonetheless, I've done nothing to discredit your theory, simply made a parallel observation which actually goes some way to supporting yours, circumstantially. Coincidentally, Europe is doing a positively spritely 5.6 knots as I write this - the fastest she's been in 36 hours from my observations! Anything could happen yet...