Ok after looking up statutory interpretation, it seems there are various rules the judge can use for interpretations, but it is up to the judge to decide which rule to use. And among these rules, the only way airsoft is considered air weapon is if it is taken using the Literal Rule and selectively with certain English dictionaries and not others, and very selectively on words, and without considering the generally understood conventional classifications. Meaning even with this method the judge will have to quite purposefully go out of his way to interpret airsoft as air weapon, which should never happen.
- Paperback Oxford English Dictionary says "Air rifle" (or "air pistol") is defined as: a gun which uses compressed air to fire pellets. And then "pellet": a piece of small shot or other lightweight bullet. - Origin Old French pelote 'metal ball'.
- Cambridge Dictionary online says "air rifle" is: a gun [...] uses air pressure to fire a pellet (= small metal ball).
And when using other methods i.e. Golden Rule or Mischief Rule or Purposive Approach, then airsoft should logically be excluded. The exclusion of "airsoft" (as a class of barrelled weapon with certain ballistic characteristics) from "firearm for the purposes of this Act" should extend to "firearm [and shot gun and air weapon] ..." because that should be the more accurate meaning within context, and for the purposes of the introduction of the exclusion as a general exclusion to the Act.
And generally, I think any judge would select the easiest method of interpretation, and the "general exclusion" method is the most straightforward, and would require the least interpretation gymnastics from the judge.
To say "airsoft is air weapon" would require the judge (actually, firstly, the prosecutor) to make a technical judgement to explain how a particular airsoft rifle uses compressed air or air pressure therefore it is air weapon, and why metal pellet is irrelevant to such a definition. Which should be easily defended with above by the defendant (or his solicitor).
So I guess we should be safe for now.