Because even if it was successful from a national security standpoint (semicon independence) it would look like a total boondoggle. It would cost billions of government money, and then overrun those billions in cost. Chips would still be slower than their competitors, and the new British fabbed chips would be even slower. No matter which party pulled the trigger, the other would ruthlessly tear the project down in the eyes of the public as corrupt and expensive, terrible for the environment, huge water and land usage, etc.
I'm not trying to say that ARM is incompetent, but it takes years for an institution to learn how to do this. ARM doesn't currently operate fabs.
It would be a separate effort from ARM itself, merely complimentary. It wouldn't have to be too expensive either, if it avoided private profit-seeking suppliers and instead attempted to vertically integrate by nationalising related industries.
You're correct that neither main party would do this, partly because of the inadequacies of the liberal election cycle. Only direct worker democracy could reliably centrally plan for use rather than profit.
I'm not trying to say that ARM is incompetent, but it takes years for an institution to learn how to do this. ARM doesn't currently operate fabs.