When optimum curing temperature is advised as 20 degrees.....what happens if below? Does Epoxy still cure but just take Longer? Or will it kill the chemical process?

I can’t guarantee that my workshop will stay anywhere near 20, but can easily leave to cure for several days.

Yes and no. It will cure, but not to the state it would at 20°. If it is much colder, you could end up with an super brittle resin, that falls apart during the demould. "Postcuring" at 20° will result in a normal curing state.
The real problem is, that most epoxys become very thick below 20° wich makes air bubble free lamination almost impossible.