U.K. Knee pads
Knee pads are a great addition to your kit in the rough rocky terrain typical of the AO. You're looking for knee pads that will be comfortable to wear under your trousers, not on top like U.S. Army troops. Basketball knee pads or BMX G-FORM type pads are typically designed to be worn next to the skin (unlike tactical knee pads). Standard BMX knee pads incorporate the hard shell popular on tactical knee pads, the G-FORM kneepads incorporate Reactive Protection, that is hardening upon impact.
If you decide you want to keep it old school, it is quite common to see OG gaberdine sewn into the lining of trousers to allow roll mat material or FFD to be inserted as a knee pad. While this won't be as effective against stabby rocks it is better than nothing.