Anything can be classed as on offensive weapon – for example a tin of oven cleaner spray or cheap nasty perfume if the intent is to use it as a weapon. Intent is a complex legal area, ultimately left to the magistrates/jury to decide, however the police can still arrest if they have good reason to suspect you're carrying it as a weapon – if you were carrying a can of oven cleaner home from the shops with your shopping, you'd be in the clear – if it had been sitting in your handbag for six months and you're stopped at ten at night walking home from the pub, then you're going to get nicked.
Pepper or CS spray however are classed as prohibited weapons, you need a special licence, or other specific home office authority (such as being a member of the police on duty) to carry it – the police don't need to prove intent, found with it, you're nicked.