Although I don't collect British or Allied WWII era patches and my main area is WWII US made patches. There is one thing in common , know your history of the unit and the history of its insignia and use as much reference material as possible. All Airborne or Special forces patches have been reproduced or made for rein-actors for quite a long time now , maybe going back 30 years so they can look the part. The same mistakes are made by collectors of US Shoulder Sleeve Insignia , especially elite units. Can cost a lot of pinger for a repro if your not careful.
Having said that I do want to collect certain UK units for a display which will include elite units so hopefully i won't fall into the trap!!