I have read and heard in numerous documentaries and books that commandos used their original units cap badge on their berets, and I was wondering if I could use my Grenadier Guards cap badge which I got when I was in cadets as that was my unit before I left?
For most the army commando like 4 commando etc yes.
2 commando had a unique badge
Many just didnt wear a badge, the commando green beret wasnt *that* universal either in WWII. early commandos wore their original headgear or camp comforters on operations and d-day and beyond many wore steel helmets. A lot of the pics of the commandoes going ashore on d-day have them in helmets.
But in answer to your question, yes that would be fine.
Probably worth looking at some archive pics of your chosen unit though and seeing what they did.
There is a fantastic commando pics archive online somewhere, i think if you google for the 'commando archive' you may find it.
"I think we are in rats' alley - Where the dead men lost their bones."
Thanks, I knew that many didn't bother with cap badges but my green beret looks plain and my cap badge is just sitting around collecting dust, but I'm going to have a look at the archive now 
http://www.specialforcesroh.com/imagelist-16.html
Images of army commando
"I think we are in rats' alley - Where the dead men lost their bones."
Thanks 
There's a lot of useful info and some great photos to be found on the Commando Veterans site as well.
