SA80 has crap ergonomics and a good one is expensive, the fn2000 is suprisingly bulky as in wide and quiet a few people i have spoken to found the grip angle uncomfortable ( not personally but i find the P90 really comfortable to hold). AUG ergonomics are all right the cross bolt safety is not really ambi but most manufactures put the notch in it to make it a fire selector as well, the grip angle is good, the trigger guard means you can wear large gloves, folding foregrip is very useful, the hop is a well designed rotary and the gear box is a standard V3. the major selling point of the AUG is it can be field stripped and the barrel can be changed quickly (granted you then need several outer and inner barrel assemblies) not majorly useful but it is cool and good for transport. cons it does have a long trigger pull but that's not really a problem and mag changes aren't as easy as an armalite style gun.
So out the three i'd get a P90 red dot with several MAG 170rd mid caps. failing that Civilian version of the AUG( the built in scope on the military models is crap no matter what manufacturer).