Because these terms are used in section 14 http://www.legislation.gov.uk/ukpga/1986/64/section/14 used to arrest those protesting.

Now I've tried looking for definitions.

I can find that a 'reasonable person' is a legal fiction.......I can sort of understand that, but I cannot see that this applies to a person (who will be definable......the senior police officer). Surely this means that it is not possible to define whether the senior police officer 'reasonably believes' or not, surely that is a totally subjective matter?

also 'serious public disorder' ......and 'serious disruption to the life of a community'.......when does 'public disorder' become 'serious'? again this must surely be a totally subjective concept.

I understand that in contract law wording where the meaning cannot be determined is unenforcable, but what happens in criminal law with undeterminable wording.

On a further matter, If someone comes at you, when you are peacefully doing something and says you cant do something because of $kl>az of section ****!!))(( act of 99999, are you allowed to question whether that is the truth and ask them to show you the relevant documentation/legal definition of it? or are you compelled to believe everything that someone tells you (this would only apply where it is likely that people could not work out that what they were doing was wrong)