Lol, nobody is using airsoft pistols for self-defence. This thread was purely hypothetical, and besides, carrying even an airsoft pistol is illegal (any replica gun, not just airsoft).
Carrying a derringer or a knife without an excuse is illegal. When I have good reason, I carry a knife, but I never carry a gun unless I'm on my to or from the range (and even then it's locked up in the back of my car). When I have no reason to carry a knife (which is most of the time), I carry a maglite as a kubaton, which is a bit studier than a roll of coins (and also more common, no one really has rolls of coins in the UK). And the only time I've been through a metal detector was at the airport...