It's a sad fact that from the beginning the priests of the GB District have worn suits in public. I don't know why. One reason I heard was that otherwise people would think they were High Anglicans!
What you heard is true; I also heard it from continental European SSPX priests (who wear cassocks) that they observed what you say when they visited the island England. They told me, that when it is important to be distinguished from Anglican "priests" who traditionally wore cassocks, some traditional Roman-Catholic priests who normally wear the cassocks, temporally used the clergyman.
That could be the reason why Bishop Williamson on a few occasions wore the clergyman in England, for example near his former place, the SSPX headquarters' house. Of course there are sometimes other reasons, too. Maybe he could write an Eleison-Comment about it, or better a series of Comments.
Now in order to make this extremely important discussion with so many extremely wise guys more interesting:
Who could post that photograph which shows Archbishop Lefebvre during a visit to his US SSPX district wearing a clergyman ?
(Hint: the photograph exists. And on it, the Archbishop looks very much like Bishop Williamson looks like during the interesting London Forum lecture. They were very close.)
One last point (I promise), just like Archbishop Lefebvre who, on occasion, wore the clerical suit.
Yes, indeed. :-)
God bless Bishop Williamson.