It would be only fractionally more sensible to use an SA80 in a crowded street than a SLR. If you'd actually been taught any shooting by the police or army you'd know about having to check the background and knowledge of when not to shoot.
For those like Walter who haven't: it is far far harder than the movies. People don't fire just once and in practice most rounds will miss and even the ones that hit can and will go though the target and innocent bystanders.
Fantasies about shooting terrorists or being a 'good guy with a gun' have little connection to reality.