The UK has particularly strong (indirect) restrictions on self defense. Askthe.police.uk appears to be an official police agency. As a police agency, they can only give their version of what the law is, but they could be mistaken. They say "The only fully legal self defence product at the moment is a rape alarm". This by itself does not mean that pepper spray and the like are definitively illegal:
There are other self defence products which claim to be legal (e.g.
non toxic sprays), however, until a test case is brought before the
court, we cannot confirm their legality or endorse them. If you
purchase one you must be aware that if you are stopped by the police
and have it in your possession there is always a possibility that you
will be arrested and detained until the product, it's contents and
legality can be verified.
One can infer that they somewhat disapprove of pepper spray:
There are products which squirt a relatively safe, brightly coloured
dye (as opposed to a pepper spray). A properly designed product of
this nature, used in the way it is intended, should not be able to
cause an injury.
The underlying theory seems to be that the dye will frighten the assailant so it might be useful. Nevertheless, they do not fully endorse spray dye:
However, be aware that even a seemingly safe product, deliberately
aimed and sprayed in someone's eyes, would become an offensive weapon
because it would be used in a way that was intended to cause injury.
This underscores the point that "intent" determines the criminal nature of the act. If you accidentally spray a dye into someone's eyes, that probably would not make the thing an offensive weapon. Moreover, if at the moment of defending yourself with dye you intentionally spray it into someone eyes, that does not make it an offensive weapon (see below on per se offensive weapons). The difference between pepper spray and dye lies in the outcome that you expect, that pepper spray will cause actual and non-trivial physical discomfort, and it's foreseeability (the point of having pepper spray is to injure).
The police are not making any definitive "rulings" (only a court can make a ruling), and they warn
The above advice is given in good faith, you must make your own
decision and this website cannot be held responsible for the
consequences of the possession, use or misuse of any self defence
Possession of other weapons (mostly knives, also weapons for beating people) is more clearly illegal, due to numerous acts enacted by Parliament over the years. The gov't. prosecutor offers useful details on their (current) policies and the underlying laws. The underlying authority for these restrictions seems to be the Prevention of Crime Act, 1953, which outlaws having an offensive weapon in a public place, and an offense weapon is simply defined as
any article made or adapted for use for causing injury to the person,
or intended by the person having it with him for such use by him
A brick or an egg could be an "offensive weapon", if a person intends to use it to cause injury. It is more difficult to see how an egg could cause injury, but actual injury is not required under the law, only intent to injure. It is thus a bit surprising that the police would be so bold as to say that a "rape alarm" is fully legal, but this may refer to a specific thing, the "Personal Guardian", which silently notifies the police, and is not a loud whistle (which could injure a person).
Intent being crucial to the determination of "offensive weapon" status, CPS points out that
where a person uses an article offensively in a public place, the
offensive use of the article is not conclusive of the question whether
he had it with him as an offensive weapon within section 1(1) of the
Prevention of Crime Act 1953.
If you use a chain or stick offensively, that does not establish that you had it with you as an offensive weapon. You crucially had to previously intend to use it as an offensive weapon: as they say:
Having an article innocently will be
converted into having the article guiltily if an intent to use the
article offensively is formed before the actual occasion to use
violence has arisen.
There are a number of per se offensive weapons:
those made for causing injury to the person i.e. offensive per se. For
examples of weapons that are offensive per se, see Criminal Justice
Act 1988 (Offensive Weapons) Order 1988, (Stones 8-22745) and case law
decisions. (Archbold 24-116). The Criminal Justice Act (1988)
(Offensive Weapons) (Amendment) Order 2008 came into force on 6th
April 2008 with the effect that a sword with a curved blade of 50cm or
more (samurai sword), has been added to the schedule to the Criminal
Justice Act 1988 (Offensive Weapons) Order 1988
but sticks and chains would not be included.
Spices are not likely to be shown to have a per se purpose of causing injury to others; but carrying pepper powder with the intent of throwing it in someone's eyes (for whatever reason) and thus injuring them fits the definition of "offensive weapon". Pepper spray even more clearly fits that definition (you don't use pepper spray in curry), and has resulted in arrests. In fact, the Firearms Act 1968 (S5) (b) specifically makes it illegal to possess
any weapon of whatever description designed or adapted for the
discharge of any noxious liquid, gas or other thing