Gareth,
the original system on PI cars has a cable which splits into two, one to the metering unit, the other to the idle cam, which both enriched the fuel and opens the throttle, so if you have an original system this does as you ask.
My car with modified throttle linkage only had the enrichment cable, which is why I did this mod.
You may need to adjust your throttle cam setting screw to get a faster idle, this should be in your manual.
I still have the original split cable, but the throttle part is defunct, and not a solid wire so it won’t push the valve closed, I would