There's no PrereqTech in table "Governments". There is only "PrereqCivic".
Unresearchable techs are actually possible tho it requres a fair amount of UI re-write, and some gameplay lua. You can thus re-direct any civ which attempts to start researching TechX to some other tech they've not yet researched but can research.
There is no method I've yet tested or found to specify the Civic a player is "resesarching" so there's no way to do such a re-direct for civics.
For techs, the attached mod does work but is a bit kludgey and would need some rework. It creates a dummy tech that is given to Rome, and which should if I have not mucked the modifiers give the same instant effect as completing Big Ben. Problem is that it will instantly conflict with anything at all like CQUI or other UI mods writing to the same game UI-files. Plus since you cannot do dummy civics (because there's no way to keep the AI from "researching" them) I didn't proceed any farther with the concept once I tested that the idea works. Also hence why all the commented lines in the SQL file -- various experiments before I found a method that would work without CTD at game start.