Certainly not uncommon here over the years for drivers to remove roofsigns for longer distance and/or chauffeur-style work, although I'm not entirely sure what the official position is on this.
One particular driver has also tried to dispense with a roofsign even on the ranks on and off for perhaps fifteen years, but he now finally seems to have some sort of permanent sign that is reasonably easy to see (his last effort as a permanent sign was a tiny little thing that wasn't illuminated and even during the day was impossible to see from many angles because of the height of the vehicle and also because the sign was sort of lost among the roofrails.)
Personally it doesn't bother me in the slightest if drivers don't have a roofsign, as long as they aren't ranking up
but as others have said, in terms of the legalities it's essentially the same as the questions about seating and CCTV etc.
But what does annoy me is the drivers who don't have illuminated roofsigns, and quite a lot here have signs so poorly lit that in town you have to be quite close to them to work out whether in fact they are illuminated