I don't know why Public player can not build signalboxes in pak128.Britain-Ex [half-heights] (I have not tryed in other paks, but I suspect it to be the game, not the pak), but I hope this has been a mistake. Especially since players can not place signals on tracks of other players (including public) but you often want the public player to build the railways.
I will take a look at it myself in a short while, but I think it will be hard to find for someone as unexperienced as I am.
Edit: added version information
This was an error caused by copying the depot code for signalboxes without modification. I have now fixed this; thank you for your report.