I recently got some of the British SA80 mags. They are slightly heavier than USGI mags and feel much more sturdy. They are also finished in a nice parkerized coat.
As others have noted, sometimes when fully loaded with 30 rounds, they will not seat under a closed bolt. I, too, found this to be the case in several of the British mags I received.
Some have advised filing the upper edge of the mag catch slot. Others have noted that the legs of the blue/black follower in the British mags are slightly longer than in the green USGI follower.
I tried replacing the followers first, and it seemed to do the trick as the mags now seat under a closed bolt when fully loaded with 30 rounds. I also tested these mags this weekend and they seemed to feed and function well and lock back the bolt reliably.
I would try replacing the follwers with USGI green ones first before filing the magazine catch hole.