Just to add a few comments to what Brother in Arms had to say, for some clarity, here in Australia the L1A1 is the Self Loading Rifle (most commonly aka the SLR) and the L2A1 is the Automatic Rifle (less commonly aka the AR). I'm not sure if the other Commonwealth nations followed the same nomenclature.
The problem with the L4 Bren mags is the spring, it's not strong enough.
They were designed as top loaders so didn't need as much force to push the rounds down whereas the C2 and L2 30-rd mags were designed as bottom loaders and needed a more powerful spring to push the rounds up. We had orders specifically forbidding any mix & match of the mags from the two because while the L2 mag works well enough on the L4, the reverse doesn't "reliably" hold true.
Having said that, I never actually saw many proper L4 mags in use in Australia. Most of the L4 Brens I saw or used were issued with L2 mags
Also, in regards to converting the SLR to full auto, it's not that much of a problem if you "know where to look". The SLRs are built the same way as the FAL rifles, that is to say, they are capable of full-auto from the start because the trigger mechanism wasn't designed to prevent full-auto. This is the reason the "matchstick trick" worked
The indent for the full-auto setting is still built into the receiver even though the British decided to redesign the FAL as a semi-auto only rifle.
The SLR has a pin inserted at a specific place to prevent the trigger mechanism from engaging the full-auto setting and it uses a slightly longer trigger plunger than the AR. I don't know for certain if the SLR trigger plunger is the same length as the plunger used in the FAL (taking into account metric to inch conversions) but they might very well be about the same length.
I'd hazard a guess and say removing this pin is the most common way of backyard gunsmiths converting the SLR to full-auto.
(While the Aussie SAS certainly used this knowledge during the Vietnam War to convert some SLRs to select-fire, they also converted a number of the ARs by removing bipods and other extraneous features. These cut down ARs are often incorrectly attributed in books/magazines as SLRs modified in the field to allow full auto. An understandable enough mistake given the almost identical look of the two, the only certain way to identify the modified ARs from the modified SLRs is to check the rear sight)