You can defend yourself with whatever your have at hand. However, you must be able to carry it legally, such as an umbrella or a rolled up magazine. Anything designed or adapted to be a weapon is illegal. This incudes if you carry a fluffy toy and SAY it's intended for self defence. So yeah, you can't carry anything for the purpose of self defence as it becomes an offensive weapon.
In addition, you must use reasonable and proportionate force, which will be assessed by a court in the cold light of day after the event. The law really isn't on our side. Again, and this applies to the US, the best way is to be aware and avoid incidents in the first place.