Its all down to mechanical protection. This is to prevent damage to any cable. If your cable run is below the decking level then the only protection it would require would be against things like rain and perhaps frost.
Armoured cable is used when external influences suggest it may be damaged by any type of impact, or, direct puncture.
It is hard to give advise for any DIY project because we have not been given the chance to fully survey the installation. For instance armoured cable may be the best solution to your problem because an electrician would be able to foresee a problem using his or her experiences over the years.
Do not forget that under your decking is a haven for wild life, and if you use a barbecue there may be problems with falling embers, spillages and such like.
If in doubt use armoured, its not that more expensive than other cables and will last far longer than most.