I get you. Like I said I don’t carry anything as such, but reading about these kind of incidents, was just curious to see other punters thinking
While it might seem ridiculous in principle, as the other replies explain, I can see why some more diminutive punters might be thinking this - especially after horror-stories such as Fife Layla and her shitbag accomplices. But if you carry out the due diligence you always should for any punt, the chances of ending up in such a situation are pretty slim - even if it is a blind, TOFTT affair.
You could conceivably carry something that's small enough to conceal in a zipped pocket - never to see the light of day unless things got really grim. However, what would this realistically be? I realise everyone would ideally love to be Carter in the bedroom scene with the shotgun, but can't see many punters planting one of them under the bed.
Following on from what Alex said, would you really be willing to go all Kamikaze with a knife on a bigger fucker, likely holding a club or such-like? Would you even be willing to actually chib the bastard if it came down to it? Even if you were that mental, this very likely raises problems far bigger than the other worst-case scenario of losing your wallet and phone, whilst gaining a slap, even if it came to that. And even in such a case, you could always plan some other sort of revenge at a later date, that's only going to fuck them, and not yourself as well.
Point being, other than some sort of glorified pepper spray or tear gas, I honestly can't think of anything else that would be effective in such a situation. Nor would I be convinced even that would definitely act as enough of a deterrent, to some semi-psycho scumbag ready with a baseball bat.