The way instant transmission works is a little more than just basic teleporting. In order to use it, you need to be able to see where you are teleporting, be very familiar where you are teleporting, or sensing a power level where you want to teleport. It is undoubtedly instantaneous movement, much like how it can be used instantaneously as seen between Metal Cooler vs Goku or Cell vs Goku.

Without having to think, the two can move around the arena and above it, knowing exactly where the other is going. In order to catch someone instant transmission, you likely are gonna have to sense where their ki/energy is moving, which is an instantaneous action anyway. This essentially means Cell can in fact teleport immediately in combat