OP has stated openly that he was dishonest in qualifying the property to aircrew staff, who would have authorisation in the process of returning the originating property to its rightful owner. In this case, I believe the lie would serve as the act of dishonesty.
Whilst it is a subjective test, a judge or jury would determine this, and neither I or you would have bearing over this.
If I was charged for this I would certainly raise s.2(1)(a). It is a subjective test, the jury must decide what the
defendant thought. Not what they, the jury, thinks. If the defendant
genuinely believes he has the right to take it, it doesn't matter how silly that is, he is not guilty. If he believes "finders keepers" is law (which in this case it seems he
may), and he can persuade a jury that is what he thought, that is all he needs to do. He would not be dishonest in the eyes of the law.
An item may also belong to the airline crew, but that is not automatic. They must have shown an intention to control items on the plane, something like signs saying where to take lost property or regular checks. This would probably be satisfied, but it doesn't change the original point that it is what the defendant thought that would be key here.
This happens all the time, whenever you get too much change in a shop you are guilty of theft when you realise later and keep it,
unless this dishonesty point kicks in. Which means
I have to return it, as I know I have no right in law to keep it!
Like I said, I'm not saying this isn't a theft under UK law. However, there are some points a defendant could realistically raise to avoid conviction, so to say as you did that "actually, it is theft in the UK" is not necessarily true.