In both the core rules for RM2 and RMC, an instantaneous spell isn't actually instantaneous. It requires the same amount of activity as casting a spell with no preparation rounds.
In the core RM2 tactical/initiative rules. - http://www.ironcrown.com/ICEforums/index.php?action=tpmod;dl=item578 - all spells are declared and cast BEFORE any other actions are declared or performed. This means that spells such as Bladeturn and Deflections must be declared/cast in the spell phase, and then held until a target can be selected in the proper phase later on in the round.
In the RMC tactical/initiative rules, unless you are using an option for "faster instantaneous spells", then they still take 75% activity, and if you do not have that activity available, you cannot cast the spell.
If you are using a different initiative system, then you will need to consult with your GM, or if you are the GM, make a ruling on it yourself.