The L1A1 was the "inch pattern" FAL adopted by the Commonwealth countries, mainly Great Britain, Australia, New Zealand, and Canada. The SA58 is DSArms semi auto version of the metric pattern FAL, adopted by just about every other non-Warsaw Pact country in the world.
Main practical difference is the magazines, the L1A1 uses a magazine with a heavy front lug welded/brazed to the front of the mag. The metric mags used a smaller lug that was simply punched out of the sheetmetal mag shell. The metric will fit in the L1A1 receiver but will be loose as the recess for the lug is too big for the smaller lug. The L1A1 mag will not fit in the metric because the lug is too large to fit in the receiver recess.
There are other differences, such as the shape of the grip, selector, and bbl type. L1A1s were semi auto only while most metrics were selective fire. Slightly different internals too.
Of course, the rifles available to us today are built up from parts kits or pieced together with a mix of surplus and new parts. Many mix features and parts, some L1A1s use metric receivers and thus use metric mags only.